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.