The Meaning of Evolution

by Samuel Christian Schmucker

Fish are stupid almost beyond the comprehension of those who are not students of the minds of animals. Frogs and toads are a distinct step in advance, and hence their mental activities play a larger part in the process.

In the love-making of the frogs and toads the song has an important share. In each species the voice is a little different from that of any other. In our familiar garden toad we have an excellent illustration of the method common to the entire group. When spring comes an impulse seems to stir in all the toads of a neighborhood. Heretofore they have stuck faithfully to dry ground; now they start off for the water. Whether their impulse is simply to move down hill or whether they by some means detect the near presence of water, I cannot say. Certainly a new fountain on a lawn will secure in spring its prompt and full share of the neighborhood's toads. In any event the toads of a district congregate in great numbers in any pond or along the edge of any moderate stream. Within a short time their flutelike, quivering voice is heard far and wide. That this note has an attractive power over the female there is no doubt. She herself makes no effort to imitate, but the song of her mate is persistent and exceedingly sweet. I have seen a male sit upon a clump of grass and utter his love call. Before he had been singing for more than half a minute three females hastened toward him from a distance of perhaps twenty feet. Each seemed anxious to reach as promptly as possible the creature whose voice had proved so attractive. When the mating comes, the female discharges a series of small shotlike eggs which are encased in a very tenacious mucous. While they are being deposited the male fertilizes them. No sooner have the eggs, fertilized by the sperm cells, reached the water than the mucous at once begins to swell. The result is that eggs appear encased in two slender strings of jelly, each having a diameter about that of a lead pencil. At intervals of not more than half an inch the shotlike eggs may be seen. The mother toad, in laying these eggs, moves about rather restlessly in the water. By this means she succeeds in wrapping the strings about the grass and sticks of the pool. This will hold them quite safely even against a considerable current of water, should the stream rise and flood the side pools in which the eggs are laid. With this amount of care, however, the attention of both parents to the young entirely ceases. They are now abandoned to the chances of a fortune to them exceedingly unkind. A toad will lay about five hundred eggs. It is evident that on the average only two of these can attain maturity by the time the parents have died, for the number of toads does not materially alter season by season. The connecting string is made up not of nourishment for the eggs, but of a bitter mucous so unpleasant to the taste that fish are thus deterred from eating the otherwise nourishing material. This secures for the young embryo a chance to mature which in the absence of the jelly it would entirely lack. Imbedded in this mucous is the embryo itself, surrounded by a small amount of albumen and containing inside of itself a very considerable amount of yolk. This gives to the egg a volume possibly a hundred times that of the egg of the sunfish. Thus, even counting the care the parent sunfish took of its offspring, which care is very uncommon among fishes, the toad stands a distinctly better chance in life. The protection of the bitter mucous and the large amount of yolk permitting considerably larger development before leaving the egg, give to the toad a material advantage. When the toad first emerges from the egg it is amazingly like the fish. It has gills at the side of its neck and swims by the movement of its tail. Later its limbs develop, the hind ones coming first, its tail is absorbed, and it is now a true toad, ready to leave the water.

Altogether a higher state of reproduction is encountered when we reach the reptiles, which are the next higher class of backboned animals. Here very distinct developments of the process are discovered. The turtle, to use the best known illustration, may lay but twenty eggs. But she will not lay them at random in the water, as do the toads and the fish. Each egg is wonderfully fattened with yolk. This means that it is possible for the creature to develop to a far greater extent before leaving the egg than was possible in the case of the toad. Accordingly the little turtle, while it begins life not unlike a fish and goes through the gilled and tailed period, during which it is not unlike a tadpole, passes beyond this period before leaving the shell and has already acquired its full turtle characters when first it steps upon the scene. So big an egg as this would be highly nutritious and animals would desire it immensely for food. Hence it becomes necessary for the turtle to securely hide her eggs. In order to do this, she scoops out a pit in the sand in which she deposits them and here they develop. If no further provisions were made the eggs of the turtle would dry completely and never hatch. Accordingly it becomes necessary for the turtle to enclose each egg in a tough, leathery membrane, known as the shell. Because the egg is thus encased it is necessary for it to be fertilized before being laid. Accordingly the male must place the sperm cells within the body of the female. These cells swim nearly to the top of the tubes in which they are placed, and there fertilize the descending eggs. Farther down the canal the shell is secreted about the now swollen mass of yolk and white, completing the egg just before it leaves the parent.

If the evolutionist understands properly the line of descent, the birds and mammals are both the descendants of the reptiles. While there is less exterior resemblance between a chicken and a turtle than between a cat and a turtle, the real relationship in the first case is much closer than in the second. This is perhaps most easily seen in the scaly legs of both bird and reptile. Another remarkable resemblance lies in the fact that in both cases the eggs are large, well stored with nourishment, and protected by a resistant shell.

So few people know the turtle's egg that it will be better to describe that of the hen, which it largely resembles. Underneath the hard shell is a tough but flexible membrane which lies against the limey coating, except at the blunt end, where a separation between the two gives room for a bubble of air. Inside of this shell and its membrane lies the white of the egg, which is nourishment for the chick, though not nearly so rich as the yolk. This, besides the albumen which it contains, is stored with large quantities of fat. It will be remembered that upon breaking a hen's egg and dropping it into a bowl, the yolk holds together because it is enclosed in a delicate sac. As the yolk falls into the bowl there floats to the top of it a lighter yellow spot as big as the end of a lead pencil. This is all of the egg which thus far represents the chick itself. All the rest is nourishment. This disk already consists of three reasonably distinguishable layers of cells, which grow rapidly different from each other. They spread and bend and twist, forming the young chick and a set of organs which serve for its protection and maintenance during its embryonic life. Within a few days these accessory organs will have formed distinctly. Within the upper half of the yolk will be found the small developing chick, which for the first thirty-six hours of its development passes through a stage not unlike the fish, or the earlier steps of the turtle. Within a few days it becomes clearly evident that this creature is to be a bird, though it is much longer before it is clearly a chick.

This embryo is so soft that it is almost like curd in thickened milk, and could be very easily destroyed were it not for a protective device which Nature has employed. It seems necessary that it should be protected with the utmost care. The matter will be better understood if we recall a common experience. Almost everyone has tried to dissolve some substance in water in a vial. If the bottle be filled with fluid to the top and corked it is very difficult to shake up the contents. Even vigorous agitation produces little movement of the material on the inside. If we wish to shake up the solid with water the bottle must be left partly empty. The brain of a human being is protected by just the same device. If it simply lay within the skull the first fall would mash the gray substance against the side of the cavity. To prevent this calamity the bony case is made somewhat larger in capacity than the brain itself, and the space between the two is filled with a watery fluid. This serves to prevent jars and shocks. In the hen's egg the same plan is pursued. The embryo lies on the inside of a bag considerably larger than itself. This sac, called the amnion, is filled with a watery fluid. With such a protection only the most severe shock to the egg would sufficiently jar the embryo to do it any harm. The ordinary experiences of an egg leave it undisturbed.

Every living creature requires a constant supply of food and of oxygen. The embryo is a living creature, and is no exception to the rule. It needs an abundant supply of easily assimilated food and of oxygen. When the hen's egg is first laid the entire contents, with the exception of the little light-colored disk which floats on the top of the yolk, form the nourishment. The disk alone is the living organism. In the earliest stages the embryo receives its food by simple absorption from the yolk. As the chick increases in complexity the yolk at first grows swampy, with fluid trickling here and there through the more solid portions. Thin walls form about these little streams, thus producing blood vessels which cover the entire surface of the yolk. These absorb the nourishment and turn it over to the embryo. As the latter grows in size both the yolk and white diminish. The embryo soon becomes larger than the remaining yolk and is attached to it by a cord filled with blood vessels which enter the chick near the center of its body. The abdominal wall has an opening at this point. One of the later occurrences in the life of the chick, before it breaks through the egg, is to have the last remnant of the yolk and its sac slip to the inside of the abdomen, which then completely closes over it.

As yet, we have seen no arrangement for furnishing air to the chick. At the same point at which the blood vessels from the yolk enter the chick, another set of vessels pass in and out. These are attached to a large flattened bag which floats above the embryo against the upper side of the shell. This bag is called the allantois, and serves as a sort of lung for the developing chick. The shell is porous enough to allow air to pass through it. The blood vessels of the allantois take in oxygen and give out carbon dioxide through the porous shell. The blood thus altered is returned to the chick and serves its life purposes. One of the reasons why the chicken must turn its eggs in the nest is that, if the allantois remain too long in contact with the upper shell of the egg, it will become attached to it and will not thereafter perform its functions.

The embryo thus enclosed in the egg finds its protection in the fact that it is encased in a fluid contained in the amnion. It draws its nourishment from the yolk upon which it lives and the nourishment is transmitted to it by blood vessels. It draws its oxygen and throws off its wastes through the instrumentality of the allantois, which covers it over. Day by day the chick becomes larger, day by day it grows to look more like what it is to be. By the nineteenth day it appears to be complete. Its nervous organization is, however, not thoroughly developed. If removed from the shell the chick still is indisposed to stand upon its feet or to run about. If allowed to remain in the egg until the twenty-first day, the chick will be able to push its beak through the skin enclosing the bubble of air at the blunt end of the egg and get the first breath into its lungs. Now it gives a faint peep, breaks the shell of the egg, and steps out into the open air.

I have given this somewhat lengthened description of the development of the chick because of the light it throws upon the method pursued by the mammals. The features which have been described in the case of the chicken's egg could be as fully observed in the case of the turtle or any of the other reptiles. Mammals are descended from the reptiles of the Mesozoic, and whatever peculiarities there may be in their method of producing their young must be derived from the reptiles. If we wish to know how the earliest mammals produced their young, we can only judge by the lowliest members of the group that live upon the earth to-day. The most primitive of these is the so-called Duckmole, of Australia. This little creature has habits not unlike those of the muskrat. It burrows in the bank of a stream, and makes a nest at the end of the burrow, where it lays its eggs. This is one of the very few warm-blooded, hair-covered animals which still lays eggs. A little higher in the scale stand the kangaroo and the opossum. These creatures keep the egg inside of the body until it is hatched. But this happens in so short a time that the young animal is exceedingly immature and as yet unable to stand the outside air. Accordingly there is a double fold of skin on the abdomen of the mother, covering her breasts. This forms a suitable resting place into which these young are conveyed as soon as they are born and from which they do not emerge for many days. The little creature instantly fastens upon the nipple of the mother, keeping its mouth constantly in this position. At intervals the muscles of the breast force the milk into the mouth of the young, which is still too undeveloped to suck for itself. As it gets older the little opossum or kangaroo emerges from the pouch in the pleasanter part of the day and in the absence of danger. It returns to the mother's pocket as soon as it becomes cold or a cry from its parent warns it of its defenseless position.