The Meaning of Evolution

by Samuel Christian Schmucker

CHAPTER VII

How the Mammals Developed

When the idea of evolution first began to be much discussed, especially after the publication of the "Origin of Species," there were several points which appeared to be more than commonly difficult of explanation. It did not seem impossible that the various types of domesticated cattle should have descended from a common ancestor. It did not seem difficult of comprehension that the dog might once have been a wolf. Though not quite so credible, it did not seem absurd that the tigers, lions, and leopards should have once all been alike. The resemblance between these are strong enough to make the idea seem conceivable. Though men were willing to concede this much, they insisted that the great branches of the animal kingdom varied so widely from each other as to make it certain that each was a separate creation. It was particularly objected that the mammals differed so entirely from other animals in several important particulars that a special divine act was necessary for their appearance. The mammals have a furry covering entirely different from the clothing of any other animal in the kingdom, and have warm blood, which is found nowhere else except among the birds. But particularly their method of producing their young seemed so entirely different from that of any other group that here a special creation was deemed absolutely necessary.

Other young creatures are produced from eggs laid by the parent and subsequently hatched. The young of the mammals are born alive and comparatively well developed. In addition, their first food, the milk of the mother, is so entirely different from the food of any other creature that this again seemed to involve a separate creation. Gradually we have come to understand the whole matter of reproduction very much better. Minute and careful dissections of rabbits, of dogs and cats, of animals slaughtered for food, with occasional post-mortem examinations of human beings in various stages of the development of the young, leave us no longer in doubt concerning the main features of the process. The better we come to understand it the more clearly it becomes evident that in the development of the mammals we have no new procedure, but, as in so many other activities, new developments of an old process.

There are two entirely different methods by which new animals and plants may arise. One sees sometimes in the home of a friend a geranium of particular beauty, the like of which he would be glad to possess. The accommodating friend cuts a small piece from the geranium. This is stuck into poor but well-watered ground, develops roots, and eventually grows into a geranium stalk exactly like the one from which it came and of which it is in reality only a detached part.

In similar fashion, if one wants a particular kind of apple, he never trusts to planting an apple seed. Going to the tree of the variety he desires, he takes from it a small twig provided with a bud and inserts this bud into a cleft made in the young branch of another apple tree. The young bud so inserted starts up into a new branch, resembling almost absolutely, not the tree which feeds it with sap, but the tree from which the bud was originally taken.

When we wish a particular variety of potato we obtain pieces of the potato of the kind we desire. Each of these must contain an eye, which is a bud of the old potato. When the sprout appears the new plant will be practically identical in character with the plant from which the potato was taken. This sort of reproduction, in which a piece of the old parent grows up into the new generation, is called the asexual method. But one parent is concerned in the process, and the offspring are as nearly as may be like the parent from which they arose.

The gardener who wishes to obtain new varieties is not content with this method. If he plant the seed of the potato the outcome will be most uncertain. His seed must be taken, of course, from the fruit of the potato, and most of these plants never fruit. Every grower of large quantities of potatoes will have noticed occasionally, on the tops of the plant, after the flowers disappear, a globular growth looking not unlike a small tomato, but with a tendency to become purplish green in color. This is the fruit of the potato and in it are the seeds. When these are planted all sorts of potatoes are liable to start up. Most of them will prove worthless. An occasional seed may produce an uncommonly fine plant. This new variety may thereafter be propagated from the tuber, as the potato itself is called, and the new strain will be kept constant in this way. This method of using the seed for reproducing the plant is called the sexual method, because two parents coöperate in the production of the seed. The pollen came from one parent and the ovule, which after fertilization swelled up into the seed, came from another. By this combination of two individuals new varieties become quite possible. Nature seems to be more concerned in improving her strain than in maintaining her older strains. In all of her lowest plants and animals she uses the asexual method of reproduction. As we go higher in the organic world the two-parent method becomes increasingly common. When we reach the higher animals, and most of the higher plants, this plan of double parenthood, the sexual method, alone is used.

In order that we may the more clearly understand how the mammals produce their young and nourish them, we shall begin at the lowest class of the backboned animals and note how the process is there accomplished. As we pass upward through the kingdom the method acquires greater complexity. When we finally reach the mammals, what at first seemed an absolutely new process will prove to be, as is all of nature's work with which we are thoroughly acquainted, but a modification and an elaboration of some previously existing process.

Some time ago I was passing the early months of summer by the side of a lake in northern Pennsylvania. Near my tent, on the edge of the water, was a wharf from which it was possible to look down into the shallows about the edge of the lake. In early July the bottom began to take on a strange appearance. Spots as big as a dinner plate became evident because they were cleaned of the finer sand or mud which is common on the bottom. A close examination showed that each of these circular spots was being occupied and cleaned up by a sunfish. The pebbles were lifted into the mouth of the fish and driven out again with force. The water which emerged with the stones seemed to wash away the dirt, while the pebbles themselves became gradually cleaned of the green plant life which ordinarily covers them. After the process was completed each spot was saucer-shaped and free from scum and mud. Over each of these spots hovered the sunfish which made it, and round and round the fish swam. The circles thus traversed were so near each other that every now and then the occupants of two adjoining nests would meet on the border. The fish which was most nearly on its own ground would at once attack the other and drive him away. In a few days the other partner in each family seemed to appear. Now two fishes swam side by side over each nest, bringing the lower edge of their bodies comparatively close together. In this position they moved around over the pebbly bottom. The female was discharging her multitudinous and very small eggs, so that they dropped to the bottom of the nest. At the same time the male was expelling what in fish is known as the milt. In this milt are the sperm cells of the male, each consisting of a rounded head and a very slender body. These are attracted by the eggs. Pushing up against them, the head of a sperm cell, consisting almost entirely of the nucleus of the cell and carrying the determinants which were to decide one-half of its future characters, penetrated this egg and fused with its nucleus. This was filled with the determinants of the characters inherited from the mother. Of course many of the eggs, of which probably there are a thousand, must have escaped fertilization. There are doubtless a thousand sperm cells that went to utter waste for one which found an egg to fertilize. These eggs nestled in the crevices between the stones in the warm water of the edge of the lake. Here the sun could easily penetrate to the bottom and hatch them. The little fish, still guarded by one hovering parent, swam around in the water long before the yolk of the egg, containing its large amount of food, had been absorbed into the tissues of the young fish. This fatty store made the abdomen of the fish in which it lay protrude enormously. Gradually the fish grew larger and the yolk grew smaller until all had been consumed. Soon the fish began to forage for himself and no longer to demand or care for the company and protection of its parent. The little sunfish is highly favored among his comrades in having any care whatever by the parent. In the case of most fishes the female, swimming slowly over the bottom, deposits her eggs, which are fertilized by the male, which follows behind her. After the eggs have thus been laid and quickened no other attention is paid to them by either of the parents.