NEARLY four hundred years ago Pierre Charron wrote ‘La vraie science et le vraie etude de Vhomme c’est I’homme’ a phrase which Pope translated and used in his famous essay as ‘the proper study of mankind is man.’ This statement, like most other generalisations, is too narrow to be entirely correct; there can be no proper study of man unless the whole of his history, his background and his contemporaries and even their ancestors are included. That is to say we believe the proper study of mankind to be man and his world, their origin, growth and development. Surely there is no more close or fascinating theme to us than this, the history of our own selves and surroundings, a history so varied and great that there can be no one who cannot find some subject to his taste if only he seeks.
Enough has already been said to indicate the manifold fields of natural knowledge comprised in those common terms Natural History and Biology. These two terms originally meant the same thing though they have now for practical purposes acquired a slight modification of meaning. The student of natural history is usually named a naturalist, a term that at present connotes an enthusiastic (and usually amateur) worker or collector in the field. He is essentially the practical man who primarily searches for the facts and specimens and only secondarily arranges and examines them in detail. Fortunately, in England, until very recently, the race of naturalists has always been strong, and such enthusiastic workers have made remarkable and valuable contributions to science, have founded some splendid journals, and are virtually the fathers of some excellent museums. Such societies and museums, the fundamental ‘pillars ‘in the scientific edifice, were made by men inspired by Nature; men who loved the fresh air, the trees and flowers, and who, literally, found ‘sermons in stones and books in the running brooks.’ While there is nothing to prevent the naturalist from also being the biologist the latter term has gradually come to mean
something a little more professional. While the naturalist, as we have said, is usually an amateur, the biologist is generally a man who is paid to follow his bent and who has spent years in training for his own particular branch of science. He, too, may love the wind in the willows, the flash of a bird’s plumage, or the song of the stream as well as any naturalist, but his label more often denotes, or seems to imply, rather the quiet investigation of the laboratory or the scratch of pen on paper in the book-lined study; and there are many different and apparently unrelated branches to which the biologist may belong. He may be a research worker interested in cancer, a bacteriologist busy with his microscope, a geologist looking at the amazing complexity of some of the early forms of iife, or some worker on mosquito control.
BIOLOGY THE KEY TO MAN’S ENDEAVOURS HOWEVER great a subject one appreciates biology to be, few fully realise its wide ramifications. Its followers in one way or another are multitudinous, and the work and industries they are concerned with equally large. Brewing and Biochemistry, Mining and Medicine, Prospecting and Publishing, Trawling and Tailoring are only a few of the widely scattered groups which are dependent in some degree upon the work of the biologist. It is important to appreciate this, for the love of nature that the naturalist has, and which some consider to be merely a hobby, has gradually grown into the great science of biology with many branches, some of which, as has been indicated, are necessities of modern civilisation and commerce.
Without a knowledge of insects and of the measures for their control, great areas of the earth now prosperous could not be inhabited, and such a gigantic and time-saving enterprise as the Panama Canal could not have been carried out. Without a knowledge of the microscopic forms of life the conquest of disease would be reduced almost to a farce. The palaeontologist with his detailed and tabulated information of fossils and their range in time has helped the prospector to track the riches of the mineral world and to tap the reservoirs of oil. The brewing of beer, the distillation of whisky, and the making of dyestuffs, depend on a knowledge of many botanical and zoological facts. Cloths and clothing depend on investigation of the breeding of animals just as paper and books require the results of botanical research.
The making of furniture and the building of houses have followed upon a knowledge of wood and trees.
Every object we use, our health and the drugs with which we preserve it, the food we eat and the clothes we wear are all based fundamentally upon the processes of nature. The work we do in office, laboratory, or industry can all be thought out to its basic necessities, and it will be seen that natural science, and generally a biological branch of it, is at the root. Our pleasures too are constantly advancing through such research.
Thus, as one might really suspect, however busy and spectacular the business and industrial powers may be, they cannot with impunity disregard the quietly working biologist. The vast superstructure that we have evolved and erected on our ancient mother earth is part of ourselves, and surely the proper study of ourselves must take notice of all the natural factors past, present, and, as far as possible, future.
THE VALUE OF MAKING CONTACTS
Now, as we have already indicated, many may be attracted to the study of natural things for the very present pleasure such study affords. Others may see in the wide importance of the subject an outlet for professional ambition and the chance of a useful and pleasant career. Both groups will naturally wish to know how further knowledge, on more or less organised lines, may be obtained so that the pleasures of the hobby or the prospects of the career may be increased or made more certain.
In the following paragraphs we shall therefore attempt to outline the sort of education that will best suit these two divergent groups of naturalists, and we shall indicate where and how this training may be obtained.
For the non-professional person who is interested in natural history as a hobby, there are several ways in which he (or she) may develop a wider scientific knowledge out of working hours and either free of expense or at very little cost. They are so well known to many that it would seem needless to mention them here, yet some appear to remain unconscious of the great treasure houses of information that are open to them.
Sooner or later the individual naturalist will tend to specialise or, at least, to prefer one subject to the others he may still enjoy. To develop his general knowledge the most useful
thing to do is to join the local Natural History Society, or any particular branch of such a society which may be desired. In this way the naturalist is brought into touch with people of similar tastes, quite apart from the fact that most of these societies possess suitable premises with a library from which the member may borrow. The society may even manage a small museum, and above all it is certain to arrange lectures on a diversity of appropriate subjects and to organise excursions. It is wise to join such a society even if it is not close at hand, for the annual subscription is usually low and the ability to borrow books is a great convenience and soon recompenses for the annual contribution.
Many such societies have their own journals and the facility of having one’s ideas and observations published is not to be regarded lightly. However obvious this advice may be, it is an unhappy fact that even among the most famous of such bodies, die membership is declining, even although so many persons appear to be interested in the subject they encourage. Perhaps, after all, too few recognise the advantages that accrue from so little expenditure and how useful the strength of combined effort and interest can be.
Every naturalist, whether amateur or professional, should belong to one such society. If the eminent biologist may not gain much from membership at least he has the satisfaction of being able to advise, and to guide the footsteps of those who will eventually fill his place.
TREASURE HOUSES OF KNOWLEDGE: THE MUSEUMS ANOTHER fruitful source of assistance is the local museum, and nowadays the country is well supplied with such iustitu-tions. Most museums have a library accessible under certain conditions to the bona fide student. The exhibited collections give an indication of local or general knowledge systematised. Education can be obtained by examining the series and reading the labels, and especially by comparing one’s own specimens with the material on show.
The larger museums have additional facilities of considerable value, perhaps the most directly useful of which is the series of lectures given by the guide-lecturer, or someone acting in dais capacity. The Natural History Museum in London, for example, has a guide lecturer, scientifically fully qualified, who gives a lecture each morning and afternoon during the week, while on Monday mornings and Sunday
afternoons members of the Museum Scientific Staff give more advanced lectures. In this way, and free of charge, a very good outline of any special branch of natural history can be obtained. Many of the other museums have somewhat similar arrangements, the Horniman Museum in London, for example, running an excellent series of lectures on Saturday afternoons throughout the winter season.
These helpful features are fortunately not confined to London. Museums in Folkestone, Bcxhill, Leeds and Sheffield, to mention only a few, give series of lectures at times to suit the average amateur, and much assistance can usually be obtained in this way. It should not be forgotten that the curators of museums are enthusiastic men only too ready to assist those of an inquiring mind and who show some desire to take the subject seriously.
There are hundreds of museums in this country alone, and the museum is a public university, without age or time limit for its students, and above all free. No amateur (or professional) biologist can afford to neglect these storehouses of knowledge which are the results of the work of generations of enthusiastic men.
THE EVER-OPEN DOOR TO LEARNING
OTHER sources of information are the extra-mural, or extension, courses arranged by various universities. Mention will be made of these later and generally they are held only in the more populous places. If the amateur is bent on a really scientific foundation for his studies he can, of course, attend some university lectures. Nearly every university and college in the country has occasional lectures by distinguished men to which the public are admitted. Further, there are numerous evening schools where a first-class training in a subject may be had usually for quite a small fee. Summer schools in various branches of biology are frequently held at some English universities. They may last for a week or a fortnight and are not expensive.
These university and college activities will be mentioned again later, but from what has been said it will be seen that the person who is a keen naturalist has plenty of strings to his bow. The local natural history, scientific or philosophical society, however it may be named, will be delighted to welcome him, encourage him and be pleased to have his observations and reports. The same is true of the museums. Their
libraries will help his studies, and even if he is so unfortunate as to have no such aids in his locality the National Central Library in Malet Place, London, W.C.I, will allow him to borrow the books he desires through his nearest public-library.
It can truly be said nowadays that there is no bar in this country to those who wish to learn. All the seeker after free knowledge needs is energy and sincerity. Given these elementary qualities and the advice of his librarian and museum curator he can draw upon the richest stores of knowledge at a purely negligible cost. Supplementing reading, writing, and listening to lectures by his customary work in the field, any man will find a hobby that will never fail and that presents a new problem every day. He will soon learn that Nature is ever changing, ever attractive and always accessible. Here is a hobby, a pursuit, for all, of all ages, and of an allurement that will persist.
HOW TO STUDY IN SPARE TIME
THERE are many followers of this hobby who may desire to serve more fully in its cause than can be done in their leisure, and it may have occurred to them that a certificated education, or the attainment of a diploma or degree may place them in a position to give full time to their desires.
There are few universities in Great Britain which allow evening or outside students to proceed to their degrees. The most notable exception is London, where external students are admitted to its examinations and where evening education leading up to the degree standard is quite easily obtained. While it is no light task to work commercially by day and to study by night, hundreds of degrees have been obtained by this method, and the recipients now occupy splendid positions in the scientific world. As examples of affiliated colleges in London which give such training we may cite the Battersea Polytechnic, Birkbeck College, Chelsea Polytechnic, and Northampton Polytechnic.
I The fees in these institutions are moderate and the tuition excellent, but they are in London. Those who desire further particulars of them should write to the Registrar of the individual college or institute, or better still to the Registrar of the University of London, S.W.7, for information as to the most suitable evening class.
Outside London there are no fewer than 135 technical
colleges in England, 3 in Ireland, 1 in the Isle of Man, and 4 in Scotland. The addresses of these institutions can all be found in Whitaker’s Almanack, and the Registrar or the Secretary will willingly furnish the applicant with information as to costs and courses. Not all of these colleges or institutes deal with biology, but many of them will provide all that the student needs. A course of one or more subjects will lead, by examination, to a certificate which is always useful.
In addition to this form of education, advantage should be taken wherever possible of the university extension lectures. These courses are arranged by certain universities and are conducted by highly competent lecturers. Attendance over a few years will give an excellent education along the selected line, and again certificates are awarded under certain circumstances. Such courses are conducted in England under the auspices of the universities or colleges at Birmingham, Bristol, Cambridge, Durham, Exeter, Hull, Leeds, Leicester, Liverpool, London, Manchester, Neweastle, Nottingham, Oxford, Reading, Sheffield and Southampton, and by each of the four University Colleges in Wales. Information concerning the nature and scope of the various lectures can be obtained on application to the Director of Extra-Mural Studies at the university concerned.
DEGREES FOR THE PART-TIME STUDENT
THE summer schools run by one or two universities and the facilities occasionally obtainable at marine biological stations are again useful, but these are intended for people who have some acquaintance with the subject, and are more in the nature of refresher courses. The announcements concerning them are to be found usually in the advertisement pages of the scientific weekly Nature.
By a judicious selection or combination of these methods the part-time student will be able to obtain some sort of certificate, if, indeed, he does not take a degree, which will help him towards a biological career.
The claims of the various correspondence schools should not be overlooked. The whole theory of the subject is adequately taught and arrangements are made for practical work, so that by this means alone the student can proceed at a moderate cost to an external degree in Arts or Science of London University, no matter in what district he lives or how he is employed. The examinations, of course, have to be
taken in London under the very strict university regulations. The path of the part-time student is therefore fairly clear, though it will never be easy and it demands an amount of determination and hard work, which is itself an eloquent testimony to the character of the student.
We must say something, however, for the young man or woman who wishes to take up biological work as a career, and who is prepared to devote his or her whole time to the study. A certain amount of biology is now taught in the schools, so that at a comparatively early stage the student has perhaps decided on the subject that is most attractive. It need hardly be pointed out, however, that early decisions are not infrequently regretted or changed, and since most of the biological sciences are interrelated there is ample opportunity for the development of new interests.
THE BEST TRAINING FOR THE PROFESSIONAL
WHAT then is the best method of procedure? If the student definitely decides at school to go in for Natural Science then the immediate problem is the entrance examination to a selected university and the determination on the course to be pursued there, for there is no doubt that a university education and a degree are essential. The matriculation examination may usually be taken at school, though the form and conditions vary with the different universities. From a general point of view the natural science degrees will include the same subjects, the only difference being the emphasis on certain subjects that the student’s taste dictates.
It is perhaps too seldom realised that the best biological education for those with the time and money to spare is the medical degree course. Medical men spend four years or so in the study of one particular animal from all aspects, and this study is preceded by one or two years’ study of zoology, botany, chemistry and physics. A medical degree is a splendid general biological education with the additional advantage that the student has two strings to his bow. First, one of the purely biological avenues he sees in the course of his studies, and, secondly, the practice of medicine. The professional aspect of this study will be mentioned later when we deal with the question of remuneration.
Usually, however, the student will enter a university or college to study zoology, botany, geology or anthropology,
and a word or two of advice and caution may be given. In the first place the ultimate object must be a good honours degree; a pass degree, though a sign of a good general education, is almost useless in the competition for an attractive post, so that an honours standard should be the student’s goal. What class of honours he attains ultimately will depend on his natural aptitude for the subject and his industry, but a first- or second-class honours degree will see him well on the way towards congenial employment.
Then, however much one subject may be admired, the others should not be neglected. In the first place degrees are not given for three or four years’ study of one subject alone, and in the second place, there is no subject that cannot be amplified by, or is unconnected with, another. The embryo zoologist should therefore not neglect some botany which will tell him of the conditions of life in another kind of medium, while the comparison of reactions of the two types of life are interesting. To understand the working of living things some knowledge of chemistry and physics is essential and, indeed, the first science examination, by whatever name it passes in the different centres, usually makes such a combination of subjects obligatory.
In his later studies for his final examination the student of zoology will have his principal subject and one or more subsidiary subjects, which might be botany, geology (or palaionto-logy), or physiology. There are often also special facilities for a greater study of insects since quite a number of universities now have a chair of entomology, and the subject is one of great commercial and professional importance.
Those who are interested in botany should follow a somewhat similar course, giving, of course, more attention to their chief subject, and perhaps less to geology, for it is exceptional for much paleobotany to be taught in the Geological Departments nowadays. The importance of the study of fungi should not be forgotten, for mycologists are much in demand at present.
Geologists, and that includes those who study paheontology, must modify their studies according to the side they prefer. Those interested in minerals and rocks need to understand a great deal of chemistry and physics, while for palaeontologists, botany, zoology and anatomy are most valuable subsidiaries. Further, while the student of invertebrate palaeontology cannot do without zoology he will find anatomy of little use,
while the vertebrate palaeontologist will find a study of human anatomy both fascinating and of constant usefulness.
Of the other sciences which have to do with biology, anthropology is an interesting study with a strong zoological and anatomical background.
WHEN THE STUDENT SHOULD SEEK ADVICE COMPLETE information of the facilities available and of the recommended courses of study for the appropriate degree will readily be furnished on application to the Registrar of the selected university, while the student’s tutor will always give the best possible advice on the particular case. Some advice is often very necessary, for as the education develops and interests widen, new attractions come into view and the first love may be deserted. This may be a wise move but it needs reshaping of studies and an abandonment of preconceived ideas. Here it is that the experienced teacher can advise and help the student not only as to the immediate changes involved but also as to the ultimate possibilities.
As no good house is ever built on poor foundations so it is most unwise to confine one’s studies or interests within too narrow walls. Speed in graduation and a limitation of outlook are often attractions for an undergraduate, but the result is apt to be regretted later on. All the biological sciences are interwoven and a knowledge of something of them all is ultimately desirable if not absolutely necessary. Wide studies, ample experience of the different laboratory methods and above all an acquaintance with the literature of natural history and its accessibility are of great importance. Equally so is a knowledge of, or at least the ability to read, German. Scientific work without this ability is almost an impossibility.
Happy is the student, young or old, who can add to these accomplishments and qualifications the ability to write good English and the gift of drawing. His publications will be natural and valuable, and his fame more easily achieved.
Besides the degrees in Arts, Pure Science, and Medicine suitable for the persons we have in mind, several universities have additional diplomas or certificates which may prove desirable. Such for example are the diplomas in Agriculture, Archaeology, and Anthropology, of Cambridge; the diploma in Animal Biology, of Leeds; the diplomas in Anthropology, Archaeology, Biology, Biochemistry, of London; the diploma
of Bacteriology, of Manchester; and somewhat similar diplomas and certificates of Oxford and Reading.
A full list of these subjects and qualifications can be obtained from the Yearbooks or Calendars published by the various universities and also from The University Yearbook (Bell).
THE PROSPECTS OF EMPLOYMENT
WHEN eventually the undergraduate becomes a graduate, after a period of research, or perhaps immediately on qualification, the question arises as to the professional possibilities of his studies. He wants to know what outlets exist for the educated and fully qualified biologist.
First of all there are the schools. As we have already mentioned biology is coming, at last, into its proper place in the modern curriculum in both boys’ and girls’ schools. Thus teachers are required and a congenial and useful career is therefore available for those who have a liking for teaching and not averse to exercising their art upon young persons. At any rate tins opportunity does now definitely exist, though it is of recent growth. Especially good work in natural science has been done in recent years by the science masters and staff of such boys’ public schools as Marlborough, St. Paul’s, and Winchester. The salaries, of course, vary with the different class of school but are generally, from the present-day point of view, satisfactory. The positions are more or less permanent, and provide facilities for pensions on retirement.
Then there are posts in the technical colleges to which we have referred earlier. These consist in demonstratorships, lectureships, headships of departments, and professorial chairs with a salary scale of roughly ^250 to £1000 per annum. Promotion is by merit and selection, and it is possible to become even the principal or director of the institution, although it must be remembered that biology is not the only subject taught there. In these technical colleges, where evening teaching is also done, there is probably less time for original work than in a college with a less onerous syllabus, but the work presents many opportunities and the staff meet keen students.
The universities offer similar positions to those in technical colleges, but the salaries and the professional status are higher. There are quite a number of vacancies every year for which those with a first- or second-class honours degree will be
serious applicants. Obviously those with post-graduate research work to their credit will be more suitably equipped, and the opportunities for this should not be overlooked nor its value underestimated. Most of the large universities are well endowed with Fellowships, Scholarships and various research-grants which are usually sufficient to maintain the student for a year or two and to permit him to do some useful piece of work under the supervision of the head of his department. In this way lies perhaps the best approach to academic life. The ultimate end of such a career is usually a professorship with a salary of £1000 to £1200 a year. Riches will, therefore, not lie much in the biologist’s path, but it will be a pleasant life among quiet ways.
CONGENIAL POSTS IN THE MUSEUMS
IN addition to academic lines of employment there are numerous other opportunities for biologists. The museums, for example, employ many botanists, zoologists and geologists. The British Museum (Natural History) has a scientific staff of fifty-two who are qualified in this way, the salaries running from £350 or so to £1400 per annum. The entrance regulations and the competition are naturally rather severe. The principal museums throughout the country have similarly qualified persons and pay salaries of between £250 and £800. Within recent years, thanks very largely to the efforts of The Museums Association, conditions of employment in museums generally and salaries have greatly improved. The museum man has come to be regarded rightly as a well-qualified friend and adviser of the public, and consequently the applicant for a position is expected to have a good degree and a good all-round interest in his work. The Museums Association has recently put a diploma scheme into operation whereby persons who have entered the profession can be examined and granted diplomas on museum competence which, with the additional scientific qualifications the curator has, forms a very important testimonial. No longer can museums be regarded as dull institutions and the larger of them must be looked on as affording congenial employment which is suitably paid, close contact with interesting and enthusiastic people, and ample opportunities for work in the open.
In the same way some of the larger libraries call for qualified biologists, and nowadays the libraries form a very important factor in public education so that they must quite definitely
be considered among the possible sources for appointments. Museums have usually a good library which needs a scientifically-minded librarian, but there are several large libraries and special information bureaux where a biologist is necessary.
SOME ATTRACTIVE BYWAYS OF A BIOLOGIST’S CAREER
APART from these obvious forms of employment, there are many government appointments at home and abroad. The Ministry of Agriculture and Fisheries has numerous departments where those qualified in natural science are wanted. Those trained in entomology might find an outlet in the Imperial Institute of Entomology at home or as entomologists abroad, where this subject is of immense importance in agriculture and public health. Some excellent appointments are available in these fields. In botany, and especially in mycology, there are similar opportunities at, for example, Kew Gardens, the Imperial Institute of Mycology, and as government mycologists in the colonies.
For geologists and palaeontologists there are many openings in the Geological Survey and the Museum of Practical Geology, in colonial geological surveys, or with the great oil companies, and with many mining and prospecting ventures.
The hospitals, too, require trained biologists for work in zoology, botany and bacteriology. Biochemical work is now much to the fore. With so much attention now paid to cancer research there are great possibilities in hospital work both for research and for teaching.
CHROMOSOMES have been shown here as the mechanism of heredity, the mechanism that makes species breed according to their kind, dogs producing puppies, cats kittens. This may seem a little difficult to reconcile with the accepted theory that all species spring from the same parent, the first living thing. Lamarck, the earliest full-fledged evolutionist, thought that the ascent of species could be explained by the ‘inheritance of acquirement.’ The primitive giraffes, he suggested, had to stretch upwards to eat the leaves of trees, and their children’s necks were therefore a little bit longer— and so on. Even Darwin tended towards a modified form of this apparently reasonable hypothesis. But both of them worked in complete ignorance of mendelism, the theory, and of chromosomes, the mechanism, of heredity. They could not know the impossibility of their hypothesis.
Since then many biologists have tried, in all possible ways, to induce the inheritance of acquirement. And every one has failed. Practically all have now given up the attempt, since modern knowledge has shown how unlikely it is to succeed. The reason is a simple one—parents do not make their children out of the stuff of which their own bodies are made. They simply hand on, unchanged, an assortment of the genes which they themselves received.
Very shortly after the egg-cell has started to multiply, one of the cells so formed gets set aside, so to speak, and thereafter pursues its own career, regardless of the body which is developing round it.
It multiplies and ripens into the mature sex-cells, male or female, and it is these closely secluded cells which eventually give rise to sperm and ova.
Instead of looking on a parent as the manufacturer of its child, therefore, we should rather regard parent and child as different branches of the same tree. Lopping a lower limb off the family tree will have no effect on the upper branches. The point is that a parent does not hand on to its child a fully developed character, ‘acquired ‘or otherwise, but only a gene, a chemical factor that will later organise the development of a similar character—just as Britain, when she founds a new colony, does not transport a fully built town overseas, complete with drainage system and boulevards. She sends out only the men who can build a town from the raw material at hand. So the renovation of London’s Mansion House is not likely to cause a magical spring-cleaning in the colony’s Mayor’s Parlour.
You will see the point best by glancing at the honey-bee, with its marvellous group of highly developed faculties for collecting honey and doing the work of the hive. The worker, the bee who does these things, never breeds. The parents of each fresh hive are the drone, who does nothing, and the queen who lays eggs. How can the lessons learned and the muscles developed by the worker in doing her job, be handed on to the next generation through the sex-cells of queen and drone?
The hive is a good analogy, not to the ideal Socialist State (to which it is often likened) but to a single plant or animal. The workers are the mortal body-cells, while the queen and the drone are the secluded sex-cells that hand on to the next generation the immutable qualities of the hive. In the old phrase, the ‘germplasm,’ the seed of the race, is immortal and unchanging, though it builds round itself successive generations of mortal, changeable bodies to nurse, protect, and diffuse it. The stream of life flows direct from egg to egg, not from egg to parent egg. Thus the parent’s experiences can have no effect on the children.
How, then, does one species give rise to another? The trouble is that this, like most other questions in life, has no single answer. Cross-breeding, to start new types by combining the best of both parent varieties; inbreeding to develop useful qualities; natural selection, that weeds out the inefficient and forces the survivors to inbreed; sexual selection
that will only permit the vigorous and attractive to mate— these are three-quarters of the answer; and very few people realise how large a part is played by natural selection alone. But neither one nor all of these can explain how variations that are fundamentally new arise in the first place. The beginnings of an explanation have only been found in the pictures the microscope has shown us during this century.
EVOLUTION AN EFFECT WITH MANY CAUSES SOMETIMES the chromosomes do not behave. Two opposite numbers may stick together, so that in the gametes of a species that normally have 8 chromosomes, one has 9 and the other 7. At other times all the chromosomes of both armies stick together, so that one resulting gamete has double the chromosomes it should have, the other none.
A variety of similar aberrations are known; and their possessors all depart, in one way or another, from the parent species. The gigas variety of evening primrose, for instance, which is twice the size of the ordinary type, has also twice the number of chromosomes.
There is another type of ‘sport ‘which is probably more important, a ‘gene-mutation ‘—an ultra-microscopic change, that is, in some single gene. Occasionally (about one in ten thousand times), for instance, Drosophila (the fruit-fly) will produce an egg that develops into a fly with mere stumps of wings. This breeds true when mated to its like, and is recessive to the normal type. Mutations of this sort have been found in practically every plant and animal studied, though their causes are still a mystery. All that we know at the moment is that bombardment with X-rays will make them occur more often. Nothing short of that seems to have any effect at all on the ‘gcrmplasm ‘.
But biologists have now amassed enough evidence to justify summarising their general opinion thus :—
Gene-mutations and (to a lesser extent) changes in chromosome number provide new variations.
Isolation, sexual selection, and inbreeding intensify them.
Outbreeding spreads them, mixes them, and provides new combinations.
Natural selection completes the process by wiping out the older, less efficient species, and leaves only the new, better adapted varieties to breed.
In some such way, during the a?ons of evolution, a single-
celled creature gave rise to one of many cells, fish produced amphibia, amphibia reptiles—and so on down to our not-so-distant ancestor who fathered both the apes and all mankind.
SOME books TO READ NEXT ON HEREDITY BREEDING, as I have hinted in this article, is very closely allied to the other activities of the living creature. So if you want a bird’s-eye view of the whole, but with greater detail than I have here been able to give to breeding alone, you will find it in highly readable (and reliable) form in the following three small books—read in the order I have given:— 1. Life, by Sir Arthur Shipley (C.U.P.).
2. Evolution, Heredity^ & Variation, by D. Ward Cutler (Christophers).
3. Living Organisms, by E. S. Goodrich (O.U.P.).
Alternatively, read Shipley first, and then either Heredity (very short), by F. A. E. Crew (Benn) or Heredity—Mainly Human (rather long), by Eldon Moore (Chapman & Hall). Each of these five books gives a full list of others on the subject.
More technical are F. A. E. Crew’s two books, Animal Genetics (Oliver & Boyd) and The Genetics of Sexuality in A?iimals (C.U.P.). Both of them, though, are especially useful to the poultry-farmer.
THAT ‘half-breeds have the worst qualities of both races,’ is, perhaps, the commonest saying about hybridisation, and nothing could be farther from the truth. Cross the wild Drosophila with the stumpy-winged variety, and the offspring will be decidedly longer-lived than either parent stock. A mule has the toughness, disease-resistance, and sure-footedness of its donkey father, with the intelligence, size, and strength of the horse. Mate the English meat or dairy breeds of cattle with the zebu-type of India and East Africa, and the hybrids will inherit their native parents’ ‘thriftiness ‘and hardiness in difficult conditions, with a large share of the size and other qualities of their English ancestry.
The hybrid usually inherits the best qualities of both stocks, for the reason that useful qualities are generally dominant to their opposite numbers, and the offspring of a cross is therefore the fortunate exhibitor of a double set of the characters that evolution has found valuable. The phrase ‘hybrid vigour,’ indeed, was a commonplace among practical breeders for many years before the science of genetics started to explain it as the result of dominance.
On the other hand, hybrids are often sterile, like the mule, and for the very same reason that they are vigorous ! Look at it in terms of chromosomes, and you will see why. The egg-cell of the horse-donkey cross contains two sets of chromosomes which are complete strangers to each other; and since two heads are better than one, the members of a chromosome-pair tend to remedy each other’s deficiencies. But when partnerships are hastily, not to say violently, formed, there are apt, from time to time, to be quarrels between conflicting temperaments.
The chief quarrel that occurs when the chromosomes of two different species are forced into intimate co-operation is over the rates of growth of the various parts of the body. The donkey, in some ways, matures earlier, and in other ways later than the horse. The result is that the two sets of chromosomes, each insisting that its own is the better way, between them manage to bungle the delicate mechanism of the reproductive organs.
If the two species are only very distantly related, egg and sperm will refuse to unite, or will quarrel fatally at an early stage. If they are very nearly akin—as, for instance the red and white Shorthorns—perfect harmony will prevail. But hybrid vigour will not then be noticeable, since neither parent variety has any particular^ useful dominant qualities that the other does not possess.
Beyond this general rule, hybridisation is a matter of detail. Each kind of cross is different from the others, and every new one is to a large extent a gamble. The really big gamble comes, however, when the hybrids (if fertile) are crossed in their turn—or mated back to one of the parent stocks, as is often done—for an immense number of new combinations of chromosomes thus becomes possible. Think of ringing the changes on the 28 pairs in cattle, for instance! Here is the point where only the breeder of genius can rise to the occasion:
(1)by breeding large numbers, to give him a wide choice;
(2)by knowing which two or three beasts among them have just the right combination of grandparental qualities to enable him to use them as the parents of a new variety.
‘But surely,’ a critic may interject, ‘there must be some foundation for the prejudice against human hybrids? ‘There certainly is; but it is not intrinsically connected with genetics. Man is the only creature with a social tradition, and that tradition is very much opposed to racial crosses. Usually, therefore, only the social outcasts of each race are willing to break so important a taboo, and you can scarcely expect such parents to produce a good type of child. Secondly, again owing to the social tradition, half-breeds find themselves from the very outset the objects of suspicion and dislike. Whatever good qualities they may possess have little chance to show themselves.
On the few occasions when proper studies have been made of crosses that carry no social stigma, the children have been found to be as sound and normal as their pure-bred companions. The English-Chinese community in Liverpool is an example, and another is the extraordinarily mixed population of Kisar, an island in the Dutch East Indies, where the people are a mixture of Native, English, French, Dutch, and German, with a sprinkling from India, the neighbouring islands, and some negroid types !
The ‘quarrelling ‘between the opposing sets of chromosomes in a hybrid, by the way, often makes itself felt in other parts besides the sexual organs. When the Canadians tried, for instance, to cross the native bison with English cattle, the disharmony between mother and child was so great as to cause high mortality at parturition. The double experiment that followed, though, was successful. The Asiatic yak was crossed with the bison and with English cattle (Hereford), yielding healthy offspring in each case. What is more, they were fertile. But there was about them all a comic clumsiness, a disproportion of one part with another, that betrayed their hybrid origin. When the two kinds of hybrids were mated together, the combination of cattle, yak and bison in the same animal had an effect that was at least equally odd.
The same sort of disproportion occurred when the Russians crossed yaks with zebus (the humped Indian cattle). The males of this match were sterile, too; though the females were fertile when crossed back either to yak or zebu.
One biologist (Bond) has gone so far as to say that in a hybrid the two sets of chromosomes tend to keep themselves to themselves, so to speak, and each to take charge of a different side of the body—so that the left side takes after the mother’s family, for example, the right after the father’s. He has produced much sound evidence, from birds, animals, and humans, to support this view. But human racial crosses are nothing like as drastic as the animal ones mentioned; and any disharmonies are too slight and dubious to be mentioned here.
A MAN MAY MARRY HIS COUSIN—WITH CAUTION INBREEDING is the opposite of outbreeding—there is more in that truism than meets the eye—and the object of quite as much prejudice. Cousin marriage is said to be the cause of mental deficiency, insanity, tuberculosis, and most of the other ills in the medical dictionary. And, it is true, they very often follow it.
On the other hand, our cattle, horses, swine, sheep, and other domestic animals have all been brought to their present state by a system of inbreeding much closer than an occasional cousin marriage. Moreover, the Pharaohs usually married their sisters; their successors, the Ptolemies, did much the same; and there were several cousin marriages in the Wedge-wood-Darwin-Galton group of families that gave us some of our greatest Victorians.
What is the explanation of these discordant results? There is nothing either vicious or virtuous in inbreeding in itself. To repeat the truism, ‘Inbreeding is the opposite of Outbreeding.’ Instead of mixing widely different characters, it combines two sets of chromosomes whose genes reinforce each other in every way. If the stock is healthy, strong, clever, inbreeding will intensify those qualities. If it is weak and foolish, or has a number of recessive or semi-recessive defects, the results will be disastrous. The whole point about inbreeding is that it intensifies all the qualities of the stock, good and bad, known and unknown.
Will it be a boy or a girl? ‘is a question that has been V V asked by so many anxious parents that there has never been a lack of prophets, or even of people who claim to control sex—some honest, some otherwise, but all equally wrong. But since ‘Boy or Girl ‘is the same as ‘Heads or Tails,’ half the answers are pretty well bound to be right, and the other half can always be explained away !
The real controllers of sex are, again, the chromosomes, which have so far been described as being ranged in one, two, or more pairs. This rule has one exception. There is one chromosome in the cells of a male which, though a sizeable, rod-like body itself, has a partner that looks, in comparison, no larger than a blotchy full-stop. The big one of this odd pair is called the X-chromosomc, the little one the Y-chromo-some. In the cells of the female, however, the X-chromosome is partnered by another X in every respect like itself. Now, disregarding all the other chromosome-pairs, see how the sex-chromosomes (as the X’s and Y’s together are called) behave in the breeding process.
The female cell is XX and can therefore only give rise to X ova. But the male cell is XY and can therefore give rise to two kinds of sperm—X and Y. If an X-sperm fertilises the X-ovum, the result will be XX, a female; and if a Y-spcrm does so, the result will be XY, a male. Thus chance, and chance alone decides, at the moment of conception, whether the child is to be a boy or a girl.
When this discovery was first made, biologists thought that it completely explained the approximate equality of the sexes
—since equal numbers of X and Y sperm should result in the birth of equal numbers of boys and girls.
The facts, though, are not quite so simple, since boy babies consistently outnumber girl babies by about 105 to 100. Moreover, counts of dead embryos show that in man—and probably in most mammals—nearly twice as many males as females are conceived, but that the male type of organism is inherently more likely to die. After birth, the male death rate of humans is consistently higher than the female, except during the short period of puberty, and over the age of 80 there are nearly twice as many women as men. The reason for the much higher male conception rate is still a mystery. But it is believed to be due to the Y-sperms being lighter and faster swimmers than the X’s, and being therefore more often successful in reaching and fertilising the ova.
In many species—cattle, for instance—the X-chromosome in male cells has no partner at all. A bull, that is to say, is XO, instead of XY, while the cow, of course, is XX. This fact (plus some more intricate evidence) makes us think that the little Y-chromosome, when it exists at all, is a mere ‘dummy,’ which plays no part at all in the life of the cell.
Essential maleness therefore consists in having one dose of the little bunch of chemicals called X, while essential female-ness consists in having two doses. There is an intriguing exception, not yet understood, to this general rule of sex-determination. All the birds, all the moths, and a few other species are exactly the opposite. The male is XX and the female XY.
CRISS-CROSS HEREDITY: THE CURIOUS FAMILY OF ‘BLEEDERS’ CRISS-CROSS heredity is the useful, old-fashioned term for something that puzzled everybody until the mechanism of sex-determination was discovered. An example explains it best, and the human disease called haemophilia is a good one, since most people have heard of hemophilics, who are popularly called ‘bleeders.’ A true bleeder is a man who may easily bleed to death from the slightest little cut, while anything like a large wound is almost certain to be fatal. One of the sons of King Alphonso of Spain died thus, having received some slight internal injury in a motor accident. The feature of this disease that was first noticed was that it only occurs in men. Next it was observed that all the children of such men were completely free from
it. Finally, it was found, the sons of their daughters often inherited it. Thus it went ‘criss-cross ‘—from bleeder father to non-bleeder daughter, and then across again to her bleeder son.
This was never explained until the function of the sex-chromosomes was grasped, when somebody pointed out that the X-chromosome was large, and therefore probably carried more genes than those concerned with sex-determination. If so, then any abnormality in one of them must inevitably be inherited in a criss-cross fashion; it must be ‘sex-linked.’ Bleeding is an example that has been fully investigated.
All the sons of this marriage, you can see, cannot help being normal, since they receive from their bleeder father only the little ‘dummy ‘Y-chromosome. All the daughters, though, cannot help receiving the bleeder X from him. but they—for a reason to be explained in a moment—do not show the deficiency. When one of them marries a normal man, however, she produces two kinds of ova. bleeder X and normal X, and four kinds of children are therefore possible.
Why a woman does not show the bleeder gene she carries, is best understood by tracing the disease backwards. The prolonged haemorrhage of haemophilia is due to the blood failing to clot when exposed to the air (normal blood takes about a minute). This failure, in its turn, is due to an almost complete lack of a substance called fibrinogen, which gives ‘body,’ as it were, to the blood. Fibrinogen is manufactured by that physiological maid-of-all-work, the liver; and a bleeder’s liver, though it is of normal size, seems to be in some ways like the liver of a child a good time before birth —before fibrinogen is needed or developed.
A bleeder’s liver is one that has never grown up ! Why? We have seen that each of us is, in a real sense, a double personality, since we have two chromosomes (and therefore two genes) for every job of work. But two are not always necessary, since one is often enough. A normal woman, for instance, clearly does not need both of this particular pair of genes, since one is all a normal man possesses. And a
woman with only one of them is no worse off than any normal man. But if a man gets that defective X, he is lost, inevitably a bleeder, since he has no normal X, as a female ‘carrier ‘has, to compensate for the deficiency. From the word ‘go ‘his liver completely lacks one of the chemicals essential for full development.
This instance, by the way, is a good example of the long chain of complex reactions between gene and character— from gene to pre-natal liver, from pre-natal to post-natal liver, from the last to fibrinogen, from fibrinogen to bleeding. And there must be many intermediate stages as well. Every gene, you may say, does its work in the same sort of indirect way. Tracking it down is like trying to trace in detail the activities of secretary, paper-makers, printers, carters, etc., when the manager of a firm gives a simple order for a new kind of notepaper !
SEX-LINKED CHARACTERS IN CATTLE
SEX-LINKED characters are not always confined to males. One of great commercial importance, recently discovered, appears only in females—milk-yield in cattle. A medium-yielding cow carries the gene with the ‘kick ‘in it in one of her X-chromosomes; a high-capacity cow has it in both. Taking the latter, she hands on a single dose of the character to every one of her children. The bulls, naturally, cannot show their dose at all, though they carry it (in their single X-chromosome). The cows will only be medium-yielders, unless their father is of the same grade as their brothers, so that they receive a dose from him as well as one from their mother.
To put it in another way, a cow inherits her milking qualities not only from her mother, but also, through her father, from her grandmother. Up till this discovery, a great many breeders had never bothered about the sire’s effect upon the milk-yield of his daughters—and consequently were always failing to breed champion milkers.
Since sex-determination in moths and birds is exactly the reverse of what it is in other species, sex-linkage is also topsyturvy—a fact to be remembered by anyone interested in poultry-breeding.
THIS is a convenient place to deal with the layman’s perennial question, ‘Which is the more important, Heredity or Environment?’ So general a question is irrelevant; the biologist’s answer is on these lines : A living thing starts individual existence as an egg-cell—a tiny packet of chemicals, and the precise amount and nature of those chemicals decides whether it shall grow up a frog or a snapdragon, a fly or a man. That is heredity. But it would never grow up at .ill unless it had the appropriate environment.
The lesser differences between a pink and a yellow snapdragon, a cart-horse and a race-horse, a negro and a white man are just as much due to heredity as the larger specific differences. The general question about heredity or environment therefore breaks up into a hundred little questions of detail, such as :
Does the seed or the soil determine a good wheat crop? The answer to this is : ‘Both.’ Does heredity or environment make one frog bright yellow and its brother dark greenish-brown?—Environment (temperature and moisture).
Does heredity or environment make one man energetic and able, another a comatose mental defective?—Heredity (bar a few cases caused by accident or disease before or shortly after birth).
Each character presents its own set of problems, and there can be no general answer. But there are certain general methods of investigating all of them, and the simplest of these is to find out whether a character occurs more often or more fully after inbreeding. If it does, then you can feel fairly certain it is hereditary in the strictest sense—like eye-colour in man—since inbreeding intensifies all the hereditary qualities.
If the strength of a character varies in brothers and sisters (like the skin-colour of the mulattoes’ children) then, again, it is probably hereditary, since brothers and sisters have much the same environment, but not, thanks to the chromosome
dance, the same heredity. If a character suddenly crops up ‘out of the blue,’ the odds are in favour of heredity, that it is a deep-buried recessive that has at last come to the surface. But if the character you are investigating appears co?isis-tently in all the members of a family, or only varies step by step with the circumstances, then you must suspect the preponderance of environmental influence. The chief scientific trouble, though, is not so much disentangling heredity and environment, as defining and measuring ‘quantitative characters,’ as they are called, such as intelligence, vigour, disease-resistance. The attempts to do this are discussed in the books mentioned at the end.
FOR the sake of lucidity, we have so far considered only well-known or easily visualised characters, such as tall and short, black and grey, and have stuck to those that depend on a single pair of genes. But obviously such big and important characters as milk-yield in cattle, health or intelligence in man, or disease resistance in plants must usually have a much more complex hereditary basis. Most of them, indeed, depend on many pairs of genes, each perhaps in a different pair of chromosomes. One simple example (that has been several times known) will explain how this works.
A White and Negro married, producing mulatto (cafe au lait) children. One of these married another mulatto and had a large family—one child much whiter than either parent, one much blacker, and the rest an assortment of varying coffee shades. Evidently there has been no blending (or all the children would be the same colour), and you can see what has happened if you suppose that there are six pairs of genes involved (the exact number is not known) in as many chromosome-pairs.
Both the mulattoes, therefore, have six black genes and six white; but each of their children is likely to get a different assortment—one with eight black genes and four white; the second, the opposite; a third, half and half (like the parents); a fourth, eleven white genes and only one black—and so on. Straightforward cases of this sort are called cumulative genes.
The ‘rose-pea-walnut ‘series of comb-shapes in chickens is a simple example of another very common kind of complication called complementary genes, which you will find explained in the books mentioned at the end of this section.
A whole large complex of such complementary and cumulative genes, spread out over many chromosomes, must be responsible for such complicated things as the different varieties of the normal human brain, the fitness of certain plants for various soils and climates, the excellence (or otherwise) of some dogs for driving sheep, and similar characters.
Let me hastily beg you not to be discouraged by this complexity of the hereditary machine. It was necessary to mention the wheels within wheels. Now you can put those details to the back of your mind, and only keep clear about the simple main principles. After all, you do not need to understand the theory of electro-magnetics in order to turn on the radio, nor
even to be a practical wireless engineer. And complicated characters can be understood (and developed or eliminated) with no more than the outline of genetical principles. The great improvement in English livestock and in most cultivated plants was achieved by men who had never heard of genes and chromosomes.
So far, we have pretended that there is only one pair of chromosomes, each of the pair carrying merely one gene. In fact, every chromosome carries a great many genes—it is a large railway-carriage packed full of passengers—and most organisms have many more than one pair of chromosomes. For the moment, we need not concern ourselves with what happens when a pair of chromosomes carries more than one pair of genes; nor with the possibilities arising from 24 pairs of genes each in a different one of man’s 24 pairs of chromosomes. But it is impossible to understand one of the most important things about heredity if we look only at a single pair of chromosomes, so now we will take three of the human pairs for an example. It is easiest if we name them after playing-cards, so, the first chromosome is the Ace, and then come the King and the Queen, and we can suppose that one parent comes from a pack with Black backs, the other from a pack with White.
The child of these parents—call it a girl—must inevitably have a Black Ace, King, and Queen, and corresponding White cards. And we must suppose that she grows up and mates with a boy of similar parentage.
Now, when we were looking at only two chromosomes, we saw that in the ‘reduction division ‘that precedes the formation of gametes, the two chromosomes always separated from one another. And the same thing happens this time—if the Black Ace, that is, goes to the top of the cell, the White will go to the bottom. The Kings likewise will separate from each other, and so will the Queens.
But all the Black cards do not go to one end of the cell, all the White to the other. Each pair sorts itself out independently of the other pairs, so that in one gamete there may be a Black Ace, and the two other cards White; in the next a White Ace and King, but a Black Queen. There are obviously many other possible combinations.
And now, to round off things, let us find genes and characters to fit these chromosomes: Ace contains a gene governing eye-colour (Black = dark; White = light). King contains a gene governing hair-form (Black = curly; White = straight). Queen contains a gene governing jaw-bone (Black = Hapsburg; White = normal).
This grandchild, therefore, has hazel eyes, taking light from his maternal grandfather, dark from his paternal grandmother; pure curly hair, taking the genes from each of his grandmothers; and a normal chin, taking the genes from each of his grandfathers. He is a chance assortment of all the factors which his four grandparents handed on to his two parents. In the dance of the chromosomes, not only are opposite numbers bound to separate completely, but the different companies of the same ‘army ‘are liable to do so.
THE CHANCE THAT DECIDES THE NEXT GENERATION THIS is very different from the vague idea that most people have of the relation between parents and children. They wrongly tend to look upon father and mother as rather like
jugs of black coffee and white milk, respectively. The pale-brown cup of cafe an lait is the child they produce, and that child, in its turn, hands on cafe au lait to the third generation.
This is the wrong analogy. You will get the right one if you will take two packs of patience cards—let us say red and blue—and call them the gametes produced by the first generation. Shuffle them together to make a child (second generation). Now pick out a complete pack from that child, but without looking at the backs of the cards, which will therefore be a mingling of red and blue. This is a gamete for making the third generation.
Do it all over again, but this time start with a yellow pack and a green pack. Now shuffle together your red-blue and your green-yellow pack. This is the grandchild, the third generation, which will show all four colours in its gametes.
Start all over again with purple-and-gold and orange-and-silver, finally combining the ultimate gamete with that from the red-blue-green-ycllow series, and so produce the fourth generation.
It will not be very long before the shop fails to supply you with any new colours, so you will be driven to combine the final gamete of your series with one of the old colours, so that both your Aces of Spades, for instance, will have green backs— the child, that is, will be pure for the set of genes carried in that pair of chromosomes, though most of the others have come from a variety of ancestors. This is very much what happens in life itself. (The human double ‘pack,’ by the way, consists of only 24, not 52, pairs of cards; but the genes in a chromosome are very much more numerous than the pips on any card.)
Since each chromosome carries so many genes, we should expect to find that two or more characters are inherited together; and in fact a great many instances of this ‘linkage ‘are known. In Drosophila, the fruit fly, for example, grey body and straight wings go together, black body marches with curved wings. In sweet peas, the kind with long pollen grains are purple, while the red flowers have round grains. You can say that the top pip of the three of hearts controls the pollen shape, while the bottom pip controls the flower-colour. Alternatively, ‘linkage ‘may be described as two genes being passengers in the same railway-carriage. It is difficult to know the linkages in man, owing to the large number of chromosomes and the impossibility of experimental breeding.
Now that we have seen how life is handed on, we can go on to discover how all those things that distinguish horses, men, and marigolds from each other are passed on from generation to generation. To do this, we must switch on the high power of the microscope and take a look into the very private life of the cell. With this great magnification, the cell becomes quite different—just as the ground looks different when you are flying at 500 feet from what it does when you are flying at 5000.
Let us suppose that, under this magnification, we are looking at a fertilised egg—the single cell that, made out of the union of the father’s sperm and the mother’s ovum, is the starting-point of the new colony. A short time after the sperm and the ovum have united to form the fertilised egg, you will see, more or less in the middle of the egg, a pattern that looks rather like two old-fashioned armies forming up in battle array opposite each other. The two armies, though, are exactly like one another. The first company of each is a
straight line, the second company of each a half-moon, the third is curly, the fourth a sort of blotch—and so on.
These companies are known as chromosomes, and you may fairly look upon them as the all-important organisers of cellular activities. You can here disregard the rest of the cell (cytoplasm), and fix your attention solely on these chromosome armies, particularly remembering that they are exactly the same size, shape, etc., and that one of them is the father’s contribution to the new individual, the other the mother’s. This is important because, as you will see, it is one of the things that show that father and mother play exactly equal parts in the making of a child.
Now, if we were looking at a human egg just after the union of sperm and ovum, we should see 24 chromosomes in each of the two ‘armies ‘—48 in all; if it were the egg of a certain kind of fly (Drosophila), the numbers would be 4-8, of the green pea 7-14. The numbers, shapes and sizes vary with each species—of which, in fact, they are the cause. We, however, need not consider these specific differences, and can pretend that each army is composed of one company only— that on the right from the father’s sperm, that on the left from the mother’s ovum.
The next thing to happen is rather startling—each chromosome splits neatly in half all the way down the middle (B). After that, the halves separate completely (C) and, following this, the rest of the egg starts to pinch together (D), finally becoming two separate but attached cells (E). The last stage is exactly the same as that of our one-celled creature just before it splits completely in half; but this time the greater magnification has enabled us to see the chromosomes.
We are not now looking at a one-celled creature, however, that splits into two entirely separate individuals, but at the first stage of a many-celled creature’s life. So the two cells, each the identical twin of the other, remain stuck together; and the next step is for each of them to divide again in the same way. Thus one cell becomes two, two become four, four become eight—and so on until, when the number has reached many millions, the fully grown human being (or other creature) is perfected.
This is a compressed description of cell-division, which is the basic principle of both growth and breeding. The main characteristic (setting aside some exceptions that are momentarily unimportant) is the behaviour of the chromosomes,
since, though the rest of the cell is very often unequally divided, those dictators of development are always halved with great precision—each of the two joint managing directors of the firm splits into two whenever a new branch is to be started. This means that in every cell of your body and brain there lives and moves that fatal pattern your parents stamped in the egg from which you sprang.
HOW WE HALVE OUR HEREDITY
Now return to the beginning. The chromatin (chromosome-stuff) of a one-celled creature splits neatly in half, and the rest of the cell makes a very bad attempt at doing the same. The badness of the attempt, though, does not matter, since the chromatin, like any good board of directors, can soon collect and organise local supplies; so that before very long the two new creatures are exact duplicates of each other and of their joint parent.
The growth of a many-celled creature differs only in that the cells remain stuck together, instead of swimming apart, and that the cytoplasm (but not the chromosomes) of each cell specialises in one particular job—forming the chalky substance of a bone or of a lobster’s shell, the soft, porous substance of our lungs, the elastic quality of a muscle fibre, and so on.
Now if a one-celled, a-sexual creature splits in half in order to ‘breed,’ it must obviously give exactly half its
chromatin to each of its two ‘children ‘—splitting both ot its chromosomes. But what would happen if many-celled, sexual creatures did the same? The ovum would carry two chromosomes, so would the sperm, and if the two united, the resulting egg-cell would have four chromosomes. In the next generation there would be eight, in the one after, sixteen, and so on until the egg-cell was carrying an infinite number of chromosomes. Many-celled, sexual creatures had therefore to invent some way of cell-division different from the ‘breeding ‘of the one-celled creatures or from their own bodybuilding process.
Instead of doubling the chromosomes, as in ordinary cell formation, they divided them in making gametes (a convenient term, meaning ‘marrying cells,’ to cover both ova and sperms). Now switch the microscope on to the reproductive organs of any of the higher creatures, male or female, just as ovum or sperm is being formed. The first picture you will see is the familiar one of the chromosomes ‘setting to partners,’, but from this point the dance is different. Instead of each chromosome splitting neatly in half, the two waltz away from each other (B), then the rest of the cell pinches in between them (C) and finally, the two halves of the cell become completely divided (D). There are thus two ‘marrying cells ‘or gametes, each with half the number of chromosomes of its parent.
Though ovum formation and sperm formation differ in certain minor ways, the description just given is in essence true of both of them.
Since an egg-cell, which is the start of a new individual, is formed by the union of,sperm and ovum, four different kinds of egg-cells are possible—AC or AD, and BC or BD. That cell divides into two by the ordinary splitting process, where each chromosome is neatly halved. Those two split again into four, and so on—each new cell being BD, like the original egg-cell. Thus the new individual possesses in every cell of his body and brain one chromosome of his mother’s two, and one of his father’s two.
Now going backwards a step, the chromosome from the mother came from her father, and the chromosome from the father from his father. Therefore our new individual has inherited the chromosomes of its two grandfathers in this case, though the combination might equally well have been AC, AD, or BC—both grandmothers, maternal grandmother and paternal grandfather, or maternal grandfather and paternal grandmother.
And now going forward a step, when our new individual comes to the point of breeding, it can hand on to its child either the chromosome it received from its mother, or the one from its father; but not both. Its gametes or ‘marrying-cells ‘will each be a chance-chosen half of its two parents’ chromosomes.
This is the essential mechanism of heredity, as the microscope shows it. But the theory of heredity, a sort of prophecy of the microscope’s revelations, was discovered many years before (in 1865) by the Austrian monk Mendel, who deserves to rank with his great contemporary Darwin. Unfortunately neither knew of the other, and Mendel’s combination of careful breeding experiments with sound reasoning was unappreciated for over thirty years.
HOW MENDEL SAW HEREDITY AT WORK
MENDEL actually worked on the green pea, but since he chanced on a minor complication (only later understood) it is easier to understand his theory by taking an example
from the farmyard, meanwhile pretending that Mendel himself is at our elbow. There are two colours of Shorthorn cattle, red and white. If you mate a red with a white, all the calves will be roan, a colour resulting from an intimate mixture of red hairs and white hairs. (This, you will notice, is rather like the microscope’s picture of an individual with one of its mother’s and one of its father’s chromosomes in each cell.)
Now comes the interesting part. If two roans are mated, their calves will be : ONE RED Two ROAN ONE WHITE
The Red, if mated to another Red, will breed true; and so will the White, if mated to another White.
The two Roans mated together repeat their parents’ performance (1 Red, 2 Roan, 1 White). This seems rather confusing until you argue, as Mendel did :—
It takes two parents to make one child.
Therefore each gives half of the essential birth-factors (‘genes ‘as we now call them.
Mendel put forward this gene theory as the only reasonable interpretation he could find of the results of his breeding experiments, and he did not do so until he had tried it out very thoroughly with several different pairs of contrasting characters, getting the same result each time. Then he prophesied what to expect from crossing Roan with Red and Roan with White,1 and the subsequent experiments confirmed him. And he was again justified when he tried crossing two and three pairs of contrasting characters at once. His theory worked.
Without considering these complexities, though, we can now go farther than Mendel himself, partly because many biologists have since confirmed and extended his experiments, using all sorts of plants and animals, but mainly because the chromosomes which the microscope has since revealed behave in exactly the same simple way as his theoretical genes.
Mendel said that the two parents must each give a gene to the child. The microscope shows that they each give a
chromosome. Mendel said that the two genes do not get mixed up together, but part company when the child breeds. The microscope shows that the chromosomes do retain their individuality, and that each passes into a different gamete.
In fact, if you will compare the picture of chromosomes in this section with Mendel’s pictures of genes, you will find that they are interchangeable (though Mendel, of course, did not even touch on what happens to the genes in the business of body-building. That is purely the discovery of the microscope).
We can now put the whole thing in a nutshell :—A living thing is, in a very real sense, a dual personality, since each of its cells contains one chromosome from its father and one from its mother. Its method of breeding is to break off a living fragment of itself, a half cell containing either its mother’s chromosome or its father’s—a matter of chance—but never both. That living half-cell unites with one from the other parent, and the new individual thus formed starts on its career with the full complement of two chromosomes, one from each of its parents.
If the characters of the parents contrast, like red and white, then they will struggle for mastery in the child, as it were, and produce an apparent blend such as roaning. But they never effect a permanent blend; and they part company when the child itself breeds. A roan cannot hand on roan to any of its children, but only a red gene or a white. The roan child of roan parents does not inherit the colour from either of them; it takes red from its father and white from its mother (or vice versa), and so compounds the mixture afresh.
There are a great many simple mendelian characters known in all forms of life. In men, for instance, if a pure blue-eyed man mates with a pure dark-eyed woman, their children will all have eyes of the hazel or light-brown type. And two hazel-type people will have one blue-eyed child, two hazel, and one dark. Curly hair mated to straight hair produces wavy; and two wavy-haired parents will have one curly-haired child, two wavy-haired, and one straight-haired.
This ‘simple mendelian ratio ‘of 1:2:1, by the way, only holds when large enough numbers are bred, since the shuffling of the chromosomes is a matter of chance, which only works out evenly in the long run. If you and a friend,
for instance, tossed pennies a hundred times, you would get this result—as near as no matter : 25 both coins Heads,
25 both coins Tails,
50 one a Head and the other a Tail. This is exactly the same as the ratio of one Red : two Roan: one White. But if you only tossed four times, you might very well get quite different proportions. It is the same with parents who only have four children. So do not count on these averages to enable you to prophesy the result of any one mating. They are useful mainly in revealing the mechanism of heredity, and next in allowing accurate forecasts of a large series of matings—for instance, in most kinds of practical breeding.
THE CHROMOSOME’S ULTRA-MICROSCOPIC PASSENGERS BEFORE going any further, it is as well to get a clear idea of what genes and chromosomes really are—since they are not the same thing. A gamete (ovum or sperm) is a rather compact living jelly of complex chemical nature. The chromosomes in it are tougher, more opaque, and even more complex chemically. If the cell is killed and stained, they show up like worms of various shapes and sizes.
Genes are, strictly speaking, theoretical, since they are too small for the most powerful microscope. But we know not only that they exist and behave exactly like the visible chromosomes, but also that they are carried in the chromosomes— many genes in one chromosome. And sometimes we are even able to say, for instance, that some particular gene is carried seven-tenths of the way down the third chromosome. The chromosome is a railway carriage, and the genes are its ultra-microscopic passengers.
It is also worth noticing that the chromosomes of an egg-cell bear no more resemblance to the red or white hairs of a cow than the directors of an aircraft company look like an aeroplane. These tiny bundles of chemicals are not animals or plants in miniature. They are the directors of a firm that builds such creatures.
DOMINANCE: THE SECRET OF ‘SKIPPED GENERATIONS’ WHEN Mendel experimented with his green peas, he had the bad luck to happen on the minor (but very common and important) phenomenon called Dominance, which con-
fused him and later biologists for a long time. He mated a tall pea to a dwarf. But, instead of the first generation being intermediate in height—as one would expect—they were all tall. When these were interbred, they produced the following offspring :
One Tall that bred true
One Dwarf that bred true
Two Tails that repeated their parents’ performance.
To us, looking at things with our present knowledge, the explanation of this result is obvious, since it is in essence exactly the same as the grandchildren of the red and white cattle—
One Red (true-breeding) One White (true-breeding) Two Roan (hybrids who, like their parents, therefore do not breed true).
The same diagram of breeding performance would fit either cattle or peas equally well. The sole essential difference between them is that, in peas, the tall gene completely dominates the ‘dwarf ‘when they come together in the same bodybuilding cells, whereas in cattle the opposing characters are of equal strength.
Mendel found several other instances in peas alone of one character dominating its opposite number (yellow over green, smooth over wrinkled), and a multitude of them have since been found in almost every kind of plant and animal. In the Aberdeen-Angus cattle, for example, the normal black colour is dominant to the unfashionable—but no worse—colour red. In man the ‘Hapsburg lip ‘(which is really a slightly overgrown lower jaw) that has characterised the Spanish and Austrian royal families, is dominant to the normal mouth.
And, generally speaking, useful normal characters are more common than defects, for the very good reason that defective individuals do not usually live long enough to breed, while healthy individuals do. Indeed, the only kind of defect that has much chance of survival is a ‘recessive ‘that can be handed on by apparently normal ‘carriers.’ But for the moment, the chief practical importance of Dominance is the explanation it gives of ‘skipping a generation,’ and of the sudden appearance of a child that is not the least like its parents or any other near relative.
RED-HAIRED CHILD FROM BLACK-HAIRED PARENTS TO take an instance that must have happened innumerable times, especially in Ireland. A black-haired man marries a flaming red-haired woman, and all their children are black-haired, since pure black is fully dominant to pure red. One of these children finds a pure black-haired mate and produces a third generation of black-haired children. One of these, who has inherited the concealed red gene also marries a black-haired mate—but one who is like herself (or himself) in having a concealed red gene inherited from an equally remote ancestor.
Few people keep records of their ancestors’ hair-colour, especially not farther back than the grandparents, so these two black-haired young people are absolutely astounded by the sudden, apparently inexplicable appearance of a redheaded child.
In fact, they have only seen an example of the sort of thing that is familiar to every practical breeder and experimental geneticist. The re-appearance of an ancestral character after several generations, due to the junction of two underground streams, is a commonplace to people who keep proper pedigree records.
A recessive (as the opposite number to a dominant is called) may, of course, get lost at the very first mating. But if there are four children in each generation, the chances are decidedly in favour of it being handed on to at least one of them. This re-appearance of a recessive from the mating of two impure dominants is the most usual (though not the only) cause of the children of any species ‘throwing back ‘to some remote ancestor, or even appearing to bear no resemblance to any member of the family!
In the examples given here, dominance is complete; and that is why they (and others) misled many early biologists into thinking that dominance was an essential part of heredity. In fact, it often does not occur at all, and at other times is far from complete. Dominance may range from the completeness shown in the Tall-Dwarf pea example to the semi-dominance of dark eyes over light and the absolute lack of dominance in the Red-Roan-White cattle—with many degrees in between.
Indeed, dominance is not strictly a matter of general geneti-cal principles at all—of the way the chromosomes behave in the actual breeding process. It is an affair of physiological
detail—of the way the chromosomes struggle or co-operate with each other in the development and functioning of the body. And these details naturally vary greatly from species to species and character to character.
In peas, for instance, tallness is certainly dominant to shortness. But in human beings, shortness normally tends to be dominant to tallness. Short parents, you will notice, often have tall children; but the reverse only occurs if there has been some developmental accident, like infantile paralysis. The white of English Park cattle is dominant to other colours, though in Shorthorns white is equal to red. In man and the Aberdeen-Angus cattle, black is dominant to red, but not in horses, which have such a complication of colour genes that the only certain thing is that chestnut is recessive to every other colour. Chestnut, consequently, is the only one that invariably breeds true.1 In rabbits, the wild colour, which is really a sort of ‘dazzle ‘mixture of several colours, is dominant to everything else.
HEREDITY is not, as some people seem to think, a little imp that perversely upsets all human ealculations. And there is nothing mystical or mysterious about it either. It is simply the greatest force in life, being, in fact, the life-force, the stream of life itself.
The old saying that ‘all living things come from an egg ‘is rather misleading. It should run ‘All living things spring from living parents.’ You are alive because you are sprung from living parents; the same is true of your dog, or the sparrow picking up the crumbs outside the window, of the potted fern on the dining-room table, of the goldfish in the bowl—in fact, of every living thing you can mention, from amoeba to man.
St. Francis, when he called the birds his ‘brothers,’ probably thought he was talking good theology, while modern humanitarians seem to consider it a metaphorical statement of how we should treat the birds. They may both be right in their way. But St. Francis was more literally correct than he realised. In pure, unmetaphorical science, the birds are our relatives—so are dogs, monkeys and apes, in nearer degrees, so are fish, spiders, lettuces, and oak trees in more distant degrees.
You yourself came from human parents, and if )’ou could trace back to their parents, and so on, you would ultimately arrive at the common stock from which both men and apes are sprung. A little further back, the monkey branch joins the main stem; further back still is the common ancestor of all mammals, including your dog. Another big step, and we come to the ancestor of all mammals and some reptiles, yet further back is the ancestor of all reptiles and birds, including your sparrow —and so on, through the amphibia, such as the frog, the fish, like your goldfish, back to the common ancestors of land insects, spiders, and lobsters. And so the great trail runs right back to a single-celled creature1 which had not quite made up
its mind whether to be animal or vegetable. One of its children became the ancestor of all plants; another started the great Noah’s ark procession of the animals.
All this is evolution. You have climbed down your own family tree from the topmost shoot called Man, down the main trunk past the places where the different great branches shoot off, to the bole at the bottom, from which spring not only the plants and animals we know, but also the germs that sour the milk and improve the Gorgonzola, that give us typhoid and tuberculosis, the putrefaction of meat and our essential manures. But, since plants and animals (including men) all tike their life ultimately from the first living thing, it is essential to grasp the fact of evolution if the principles of heredity are to be understood.
THE IMMORTAL CELL
ONE-CELLED creatures and their ways have already been described in detail. Here we will look at one of them that is, in essentials, a representative of them all. We will take the ‘Slipper ‘(Paramecium). Its precise shape and other etceteras do not matter to us, and we can also disregard its real size, and suppose that it is under the appropriate power of the microscope.
This funny little thing, looking like the white of a raw egg with specks in it, swims into the field of the microscope just at the moment it is going to ‘breed.’ It does not need any partner in the business, since its ‘breeding ‘consists in pulling itself in half.
Both these two little new ‘Slippers ‘now start to feed and grow, each re-forming whatever organs it has lost—one, for instance, has no mouth, because that organ was stolen by the other. The second has to re-form the primitive stomach—and so on. Both of them soon succeed in becoming exactly like their ‘parent,’ and in growing to the same size. Each ‘child,’ in its turn, becomes a ‘parent ‘in the same way, so that there are now four ‘Slippers,’ instead of the original one— and so on indefinitely.
In fact, setting aside accidents, there is no such thing as death for a ‘Slipper,’ and the one we are looking at now is, in a very real sense, identical with the first that lived, aeons before the earliest fossils in the rocks. That great desire of so many men, Immortality, was and is the normal possession of the simplest known creatures. Death is the inevitable
penalty only of complicated creatures like a cow, a man, or a cabbage. Why? That question is answered by the next step upwards in evolution, when some single-celled creature started to split into two halves, but did not quite finish the job, so that the halves remained stuck together, like the cells of a honeycomb. These two cells split again, in the same incomplete way, making four cells all joined together. The process went on until the growth of the organism was ended. Cell-division of this incomplete kind is known as ‘cleavage.’
REPRODUCTION WHICH GIVES DEATH A MEANING THIS colony of cells all joined together had decided advantages over the old single-celled type of creature, since one group of cells could specialize in making an efficient mouth, another a high-capacity stomach, while those all round the surface could give themselves whole-heartedly to forming little oars to row the colony through the water, or to fan floating bits of food into the communal mouth.
This is the beginning of the creatures we commonly see, such as lettuces and oak trees, animals and men—highly organised colonies, in which each little group of cells has one particular job to do for the common good. The story is rather
like that of the mediaeval carpenter, who used to do everything from cutting up the tree-trunk to inlaying and polishing the finished cabinet; and who has now been replaced by an organised factory, with one man cutting the planks, another smoothing them, a third laying out the joints—and so on up to the last touch of polish. One man, one job—specialisation. And just as a factory has to have a manager, who is not necessarily good at any specialised job, so those early cell-colonies kept a few unspecialised cells which had no work to do except to pinch off halves of themselves. Those free-swimming halves became the founders of whole new colonies. When life had reached this stage of budding off the beginnings of a complete new colony, there was no longer any need for the parent colony to go on living. In fact, it had better die, to prevent over-population. Natural death therefore arrived as the cure for too much birth !
THE BEGINNINGS OF SEX
AT a very early stage in evolution these free-swimming halves became divided into two distinct sorts—a rather large, well-fed cell that floated quietly, the ovum, and a more compact, active little cell which swam vigorously about, the sperm. When these two half-cells met, they united into one, forming the fertilised egg, which thereupon started to multiply, in the usual fashion, into a new colony. At one evolutionary stage—there are many living creatures still in it—any cell colony would produce both ovum and sperm. A little
later came the next step, when one colony of the species produced only ova, another only sperm.
Here is sex as we know it to-day, with the members of a species divided into two—the females who produce the ovum kind of cell, and the males who produce the sperm kind. We can now summarise the beginning of life into three stages :
1. A single-celled creature which, when it grows too big for comfort, divides into two separate creatures.
2. Its descendant, which divides into several attached cells, each specialised for some particular job, but which occasionally throws off separate half-cells that multiply into new colonies.
3. A similar sort of colony, but rather better organised, which throws off only the sperm type of half-cell—a male colony—and a corresponding female colony that throws off only the ovum type of half-cell. Ovum and sperm thereafter combine into one whole cell, the fertilised egg, that becomes the founder of a similar colony, male or female.
There are many minor variations of this essential process, but none of them is important. The main thing is that the stream of life flows unbroken from parent to child—and the child ‘takes after ‘the parent in more respects than that of merely being alive.
MILLIONS of years ago, when the first living creatures made their appearance in this world, they were composed only of one single cell which extracted its nutriment and life-giving oxygen from the water in which it lived. In those far-off days, nothing could live outside the water which prevented it from drying up and dying and which provided a surrounding always constant in composition. As time went on and more living things developed, certain cells began to form a skin which prevented drying and enabled the organism to live in the air and to be independent of the water which previously nourished it.
Before this could happen, however, a substitute for water had to be found, and it was necessary that this substitute should bathe every cell in the body with its life-giving properties, so that each cell could continue to live. The substitute which the animals developed was blood, and the functions it serves are those which we have mentioned.
Blood is a fluid which, between the narrowest of limits, is always of the same composition; it has the peculiar property of being able to carry oxygen from the lungs and pass it on to all the living cells, no matter where they are situated, and it carries food substances for the cells, which enable them to carry on their various activities and to repair the wear and tear which work imposes on them. In addition, it carries the chemical messengers which we have discussed already, and
also certain substances which the body produces for its defence against various germs.
THE RED MILLIONS WHICH BRING LIFE TO THE TISSUES IF we place a drop of blood under the microscope, we shall find that floating in a straw-coloured fluid are millions upon millions of tiny cells. These cells can be divided into two great groups which serve quite different functions. The first group are the red cells, whose role is to carry oxygen from one part of the body to another. Each red-cell is a tiny circular disc, so small that its diameter is only seven-thousandths of a millimetre and its thickness only one-thousandth. The red-cells are so numerous that in one cubic millimetre of blood, which is a quantity considerably less than the size of a pin’s head, are no less than five million red-cells. If the number falls to three millions, we complain that we are suffering from anaemia.
The cell is composed of a thin membrane outside, with a sponge-work of the same material inside, holding in its meshes a solution of haemoglobin. This chemical substance is responsible for the oxygen-carrying properties of the red-cells. Haemoglobin has the property of combining with several substances if these are presented to it in sufficient quantity. At a given moment, it will take up large quantities of certain substances, if present in excess, and give them up when they are present only in small quantities in its surroundings.
Let us suppose that we have a single red-cell in one of the small arteries leading to the tissues. This red-cell is saturated with oxygen. When it reaches the tissues, its surroundings are nearly devoid of oxygen but highly impregnated with carbon dioxide, a gas which is produced by the tissues when they burn up food substances. The red-cell is forced to give up its oxygen, which is greedily seized and used by the tissues, and at the same time it takes up the carbon dioxide which the tissues have produced and must get rid of.
The red-cell then continues on its journey through the veins and heart and eventually reaches the lungs. Here, because the lungs are filled with air, the concentration of oxygen is high and that of the carbon dioxide is very low, so the red-cell exchanges its load of carbon dioxide for a further load of oxygen and goes upon its beneficent round once more. The function of the red-cell, therefore, is to provide the tissues with oxygen and to carry away their waste products.
TINY CELLS THAT ARE SCAVENGERS OF THE BLOOD IN each cubic millimetre of blood are about nine thousand white cells of several different kinds. The functions of some of these are quite unknown, but others have been seen to do their work under the microscope, and we know a great deal about them. One of the best-known, and one which is fairly typical of the others which do the same work, is known rather grandiosely as the Polymorphonuclear Neutrophil Leucocyte. This name describes it, for it is a white cell (Leucocyte) which takes up neutral dyes when stained (neutrophil) and its nucleus is usually divided up into several lobes and may take on many different shapes. It is known for short as a polymorph.
These cells behave in the blood and tissues exactly as does the single-celled amceba which lives in the water of our country ponds and is taken as the prototype of the commencement of animal life. If an amceba or a polymorph finds itself in close proximity to a small particle of matter, it will throw out long processes of its substance, surround the particle, engulf it and digest it. This procedure cannot be described as a voluntary one on the part of the cell; it is merely the reaction which is forced upon the cell by chemical substances exuding from the particle in question.
The amceba and the polymorph are not free agents in the matter, for they have only two courses open to them. Faced with a body which produces any reaction upon them, they will either move away from it if the substance repels them, or will engulf it, if it attracts them. It is the chemical composition of the body which determines their reaction and not any voluntary process on their own part.
The amceba engulfs a particle in its surroundings only for the purpose of supplying itself with food, but, while the polymorph may do this, its main function is to eat up and digest and destroy foreign bodies, germs and the like which might prove dangerous to the body in general. Very often the germs are so virulent that they may multiply inside the body of the polymorph instead of being digested, and in this case the cell will die in the perfojmance of its duty.
If polymorphs from the blood are placed on a microscope together with disease-producing germs, this process can be watched directly and a graphic record may be taken by the cinematograph which will show it up to perfection. The
polymorphs will engulf the bacteria and, under favourable circumstances, they will be seen to disappear under the influence of digestion. The polymorphs are thus the scavengers of the blood and represent the first line of defence of the body against disease-producing germs.
WHEN THE BLOOD CLOTS FOR OUR PROTECTION INCORPORATED in the blood are a third group of tiny bodies, smaller even than the red-cells, which are known as platelets. Their job is to assist in the formation of a clot, when this proves necessary. When small blood vessels are cut by accident, blood will pour out and, unless something is done to stop it, the bleeding will continue indefinitely until no more blood is left in the body. Clotting of the blood is the first defence which the body puts up to prevent severe bleeding after an accident, and it is also one of the ways in which the spread of infection is stopped in severe cases because, through it, the circulation is stopped in the infected area, with the result that the infecting organism dies and is cast off without the infection becoming generalised.
Clotting, therefore, is essentially a reaction to damage, and only this may start the processes which lead to clotting. How disastrous it would be if all the blood in our body were liable to clot for no apparent reason! Nature has arranged therefore that severe damage, and only this, shall lead to a clot forming in the vessels of the affected part. The whole chemical mechanism is a very complicated one and cannot be gone into fully here, but we may briefly say that three substances, including calcium, are always present in the circulating blood. When tissues are damaged and bleeding takes place, the platelets also become damaged and from the tissues and platelets is produced a substance known as thrombokinase which works upon the three chemicals we have mentioned until a clot is produced.
When a vessel is cut, clotting is the first defence against severe bleeding. Soon, however, cells begin to wander into the clot which becomes organised later into solid, immovable living fibrous tissue which ensures that no further bleeding can take place. What disastrous consequences can follow inability of the blood to clot can well be seen in the condition known as Haemophilia. This disease is a rarity and no one yet knows the exact cause of it. The essential trouble is that the blood will not clot in the normal time, with the
result that the unfortunate victim will bleed to death after the most trivial of injuries—for example, if a tooth is removed.
HOW THE TISSUES DEFEND AND REPAIR THEMSELVES IF the tissues are damaged in any way, either from violence or as the result of infection by genus, a definite train of events follows. First, as there is much work to be done by the cells in defending themselves, the blood supply must bt increased. This is accomplished by dilating the blood vessels which supply the infected part, the dilation itself being the result of a reflex which is set in motion at the site of damage.
After a time the circulation slows and large quantities of fluid from the blood escape into the tissues through the damaged vessel walls in order to dilute the poisons which the infection is producing and to carry substances which will destroy these poisons. At the same time, polymorphs actively make their way through the walls of the vessels and creep into the tissues, attracted by the chemical substances produced by germs and damaged tissues. Here they eat up and remove the germs and any tissues that may be dead. Later fibrous tissue grows into the wound which is thus replaced by a fibrous scar.
Everyone knows the signs of inflammation. These are heat, redness, swelling and pain. All these are explainable on the basis of the facts we have just mentioned; because more blood is arriving at the part, it becomes hot and red. Fluid exuding into the tissues makes it swell and stretches the delicate nerves which produce pain. Because the part is damaged, it ceases to work, and loss of function results. Thus an inflamed joint is not moved and an inflamed muscle will not contract. Rest is essential for repair, and this is automatically secured.
THE FORCES THAT KEEP US BREATHING
WE have seen how the respiration of each cell of the body is effected by the red-cells carried in the blood, and we have discussed in the section on Anatomy how the lungs expand and relax with the chest when breathing takes place. What we have not yet explained are the forces which keep breathing going on—why in fact we breathe at all. Situated in the lower part of the brain, just where the spinal cord joins it, are a number of nerve cells which control respira-
tion. These cells are connected by nerve fibres directly to the muscles which raise the ribs and to the diaphragm.
These cells, just like the heart, have the property of sending out at regular intervals impulses which travel down the nerves and activate the muscles. They exhibit, in fact, rhythm. Moreover, they are exquisitely sensitive to the composition of the blood which bathes them. We have seen that the blood takes up the waste products of combustion, namely carbon dioxide, from the tissues, and we have seen also that it carries a large quantity of oxygen picked up in the lungs. These two substances are the factors which control respiration, and they do it quite automatically. They control, increase and decrease, and even abolish the inherent rhythm of these cells which have been called the respiratory centre.
Primarily, the centre depends on the amount of carbon dioxide in the blood for the impulses it sends out to start breathing. Increase in the carbon dioxide will increase the number of impulses sent out in a given time and will increase the frequency of breathing, so that more carbon dioxide is washed out of the lungs and more will leave the blood. A decrease in the carbon dioxide of blood will result in depression of respiration, until the carbon dioxide increases once more sufficiently to stimulate the cells again.
A simple experiment that anyone who is sufficiently interested can do for himself will prove this. Breathe very deeply and at a rapid rate for, say, thirty breaths. You will then find that you will stop breathing and not recommence for quite a long while. The explanation is that by voluntary forced breathing, you have washed out so much carbon dioxide from your lungs and blood that your respiratory centre is no longer stimulated and ceases to function until the carbon dioxide content of the blood rises again to a sufficiently high level.
Oxygen has rather a different effect upon the centre, for when its blood content is low, it makes the centre more sensitive to the existing content of carbon dioxide, so that a deeper and more rapid breathing results. Lack of oxygen has only an indirect effect; carbon dioxide has a direct effect. When we climb a high mountain or go up in an aeroplane to a great height, we breathe automatically faster, for there is less oxygen in the air and therefore less in the blood, with the result that the respiratory centre is stimulated. It will thus be seen that the respiratory centre tends to keep the amount of oxygen and carbon dioxide in the blood always within the limits which
are best suited to the body. An increase or decrease in either will result in immediate and automatic compensation. Thus is the blood always kept properly aerated, and thus is the removal of some of its waste products automatically ensured. We have spoken so far only of the automatic control of breathing. We can, of course, regulate our breathing at will. The rib muscles, like any other muscles of their kind, are under the voluntary control of the brain. This mechanism, however, ensures that we shall always breathe, and enables us to breathe without having to devote our attention to it.
CLEARING OUT THE WASTE: THE KIDNEYS AT WORK THE purpose of the kidneys is to remove from the blood those waste products which have resulted from cell activity in all parts of the body; the kidneys remove and secrete the ‘ash ‘left over from the fires of life, which, if it accumulated, would clog the delicate mechanisms and eventually kill us owing to its poisonous effects. Urea and uric acid are the main substances which the kidney removes from the blood.
The tiny blood vessels which result from the division of the kidney arteries pass to an enormous number of structures which are known as glomeruli and which are situated only in the outer layers of the kidney. A glomerulus may be likened to a ball made of a very thin filtering membrane to which is attached a tube which opens into the inside of the ball. One side of the ball is deeply indented by a mass of capillaries which are coiled upon themselves and continually branch, forming a densely-packed network. Under the influence of the blood pressure a large volume of the fluid part of the blood containing the waste products is forced through the glomerular wall and filters through into the interior of the ball.
This fluid is, of course, identical with the blood fluid, and not only contains the waste products but also many substances which are valuable to the body and must be reabsorbed. The only substances which cannot get through the glomerular wall are the proteins contained in the blood fluid, for these are formed, as we have seen, of very large molecules and it is by virtue of their size that they are retained. The glomerular filtrate, therefore, is identical with the blood fluid except that it contains no proteins. This has been actually proved, for fluid has been drawn off under the microscope from a living glomerulus and has been analysed.
From the glomerulus the filtered fluid passes into the tube which leads out of it and here it starts on a long journey. The glomerulus is situated in the outer layers of the kidney and the first part of the tube remains too in this layer coiling and twisting upon itself amongst the glomeruli. Then it starts to move straight towards the centre of the kidney and having gone about half-way it turns back upon itself, forming a loop, and returns again to the outer layers where it becomes coiled and twisted for the second time. After this it turns again towards the kidney centre and joins up with many similar tubes which are each coming from a similar glomerulus and passes its contents eventually into the ureter.
During this long journey the glomerular filtrate becomes greatly changed. It is changed, in fact, from blood fluid into urine. All the substances which the body requires and must not waste are absorbed into the blood by the cells of the tubules through which it passes. At the same time an enormous volume of water is also reabsorbed by these cells so that the urine which leaves the kidney is greatly concentrated and contains only waste products and nothing which could be of value to the body. A great deal of work is therefore done by the cells lining these tubules, even if we consider only the water which has been reabsorbed. It has been estimated that, in the cat, if the glomeruli filter off twenty-four pints of blood fluid, twenty-three and four-fifths pints of water is reabsorbed and that only one-fifth of a pint of urine is produced.
Many people say that this simple account is not the whole story of how the kidney produces urine. They say that in addition to absorbing water and some solids the kidney tubules also actively secrete waste products into the fluid passing down them. Some even go so far as to state that the tubules only secrete and do not absorb water, and that it is this secretion only which concentrates the urine.
The problem of how the kidney works is one of the most vexed questions in physiology and perhaps more ink and paper have been wasted and more wordy battles have occurred on it than upon any other subject in the whole of physiology. We will leave the matter here and say only that there is good experimental evidence that both processes occur, and therefore that both parties are probably in a measure right. Anyhow it is not a matter of the greatest importance for we have made for ourselves a theory which, if not absolutely correct, is sufficient to explain the main workings of the kidney both in health and in disease, and from it many excellent methods of treating kidney diseases have been worked out.
SOME books TO READ NEXT
books about these two scientific subjects, Anatomy and Physiology, are of necessity inclined to be technical, but there are some works which may be tackled with interest and profit by the layman as well as by the medical student. Far and away the best work to read on Anatomy as a reference book is Gray’s Anatomy. It is, of course, technical, but is
profusely illustrated. Three books of great interest to the student of human anatomy are : Human Embryology and Morphology by Sir Arthur Keith, History of the Human Body by Wilder and F. Wood, Jones’s Man’s Place Among the Mammals. These three books, although containing a number of technicalities, are much more interesting than those dealing only with pure Human Anatomy.
Physiology, like so many sciences to-day, has many different divisions, and as many different books have been written about them. An interesting, simple and well-written book on the chemistry of the body is T. R. Parsons’ Elements of Biochemistry. Bainbridge and Metroes’ Textbook of Physiology is an elementary book which covers the whole field of Physiology, but being condensed makes somewhat difficult reading. For those who want to learn more about advanced Physiology Starling’s Physiology is the standard work.