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.