The Meaning of Evolution

by Samuel Christian Schmucker

These creatures are the lowliest of the class upon the earth. The great majority of all mammals have elaborated a far finer plan, in which the young are retained within the body of the parent until they are quite able to stand the air. The length of this time varies in different mammals from a few weeks to more than a year. The egg must be fertilized before it leaves the body of the parent. If it should fail in this it simply passes out and is wasted. If the fertilizing cell reaches the egg before it has progressed far down the tube it begins its development. The embryo forms for itself the sort of head and tail and gill slits which would have served its fish or its tadpole ancestor. Its limbs develop as little buds indistinguishable from similar buds that would have formed fins for the fish or wings for the bird.

Around the embryo there forms a sac, the amnion filled with a fluid which serves to protect the young mammals exactly as the growing chick was protected. Under the forming creature there hangs a small but empty yolksac. This is an actual remnant, a reminder of the past, when the eggs of the mammals were also packed with yolk and the growing embryo secured its nourishment exactly as does the maturing chick. But a new method has been provided for the mammal, and consequently the yolksac, though it has not entirely disappeared, has no nutritive content for the growth of the embryo.

The allantois of the chick now gains a new development and an altered function. In the case of the chick it floats against the shell of the egg and absorbs oxygen through the shell. Inside the body of the mammal this is impossible, because the air is too far away. No shell is formed about the egg because it is not to be laid. The tube of the parent's body in which the egg lies becomes thickened at the point of contact with the egg. It grows spongy and full of blood vessels. Meanwhile the allantois is also growing spongy. These two tissues are so closely pressed against each other that the blood vessels of the transformed allantois mesh in with those of the thickened parent wall. Thus the blood vessels of the mother are brought into close contact with those of her offspring. Her blood seeps over into the transformed allantois which is now called a placenta. From this it is handed over to the offspring, which thus receives from the mother her blood, and returns its own used blood for enrichment and purification. So the allantois of the reptile has become the placenta of the mammal. In the first instance it served only as an organ of respiration. Now it has come to supply the embryo with rich blood containing both food and oxygen derived from the mother. After the offspring is born this thickened pad breaks loose, and subsequently is also extruded from the body, forming what is known as the afterbirth.

Thus far we have spoken of the change in the method by which the young are brought to such a stage of development that they can stand the outer air. One of the improved differences between the mammals and other animals lies in the method by which they nourish their young for some time after birth. The very word mammals signifies an animal who is in the true sense of the word a mamma. This name for mother is given to her because of the fact that she possesses what are technically known as mammary glands, or, in simpler language, breasts. It would seem as if here we had an entirely new organ. No other animal gives nourishment to its young in such fashion; all mammals do. What is the origin of the habit? How did the organ arise?

A part of an animal's body that has the power to gather material from the blood and pour it out in the shape of fluid is known as a gland. Sometimes a whole organ does nothing else. Sometimes small glands are scattered through, or over, the surface of another organ. There are two kinds of glands in the skin of the mammal. The best known and most frequently thought of are those which pour out the perspiration. These have a double function. In the first place they assist in keeping the temperature of the body uniform. When we are too warm they pour out a watery fluid over the surface of the body. If the air is dry enough and our body not too closely protected by clothing, this perspiration passes off in the form of vapor. All evaporation requires heat, which in this case is extracted from the body. So soon as the temperature returns to its normal level the flow of perspiration ceases. The other function of the sweat glands is to take from the blood some of the waste matters of the body and pour them out upon the surface. This is done in order that the body may free itself from substances which, if they were to accumulate, would have a poisonous effect upon its action. It is this function of the sweat glands which makes it necessary for us to bathe the surface of our bodies with water. Dirt, in the ordinary sense of the word, is not harmful to a sound skin. Our reason for bathing is really to remove the wastes which we ourselves have poured upon the surface of the skin. These, if allowed to remain, soon decompose, like all nitrogenous substances, and become very offensive. They may then be reabsorbed into the skin and nature's effort to throw them off has been in vain. These glands, since they contain waste matter, could not possibly yield food for the young. They would poison and not nourish. Hence, whatever the breasts may be, they are not altered sweat glands.

There is another set of organs in the mammalian skin. At the base of each hair lies an oil gland. The function of these is to pour out a substance which spreads along each hair and over the surface of the body. The outside of the skin is always dead, and would easily crack were it not for the constant secretion of this oil. In winter, when the blood circulates less freely and these glands consequently pour out less oil, the supply frequently runs short. If what little is poured out is too frequently removed by washing, the skin becomes brittle, and, on bending a joint, the epidermis cracks. The gloss of the hair is due to the oil thus poured out. This oil becomes one ingredient in the milk produced by the transformed gland. But there is another important constituent. When one does unaccustomed manual work the ordinary result is the formation of a blister. The epidermis, or scarfskin, becomes detached from the dermis, or true skin, and the space between the two rapidly fills with the fluid portion of the blood, known as lymph. The fact that no blood vessels have been broken in this detachment results in there being no red corpuscles in this fluid. Wherever a cavity forms in the body lymph is liable to enter it.

The milk glands of the mammals are modified oil glands. The fluid which they now pour out is no longer exactly the old oil with the addition of the lymph. Undoubtedly in the past the first milk was more like this simple mixture. There seems no doubt that the breasts of to-day are the enlarged and modified oil glands of earlier mammals. In one of the most primitive of our mammals the young simply lick certain bare spots on the surface of the mother's abdomen. As higher forms arise there develops a smaller or larger mound with a distinct projection, about which the lips of the offspring can easily fasten. Lamarck would have said that the suction of the infant had produced such a mound, and that this had been transmitted to later offspring until it had grown to be the highly developed organ we now find, for instance, in the cow. Since, however, we have come to disbelieve in the transmission of acquired characters, this explanation will no longer serve. We must content ourselves with saying that, by whatever accident the nipple arose, the success of it when present determined its selection by nature and its consequent persistence. With increase in its function has come increase in the size of the glands. Lower animals which, like the hog, produce a large number of offspring, possess a large number also of these glands. With the diminishing number of young and greater care of them as we rise in the scale has come also a diminishing number of breasts in the female. Whether those on the front of the body should persist, or those on the rear, depends upon other factors in the life of the animal. Hoofed animals, perhaps because their best weapon is the hoof and they can there best protect their young, have retained them in the rear of the body. In the group of animals known as the primates, including monkeys, apes, and man, the habit of holding the young in the arms for protection has determined the persistence of the breasts upon the chest rather than the abdomen.

It is interesting to notice that the habit of the elephant of protecting its young by means of its tusks has also resulted in a similar position of the milk glands.

That the primates had once a larger number of offspring is confirmed by double evidence. Even to-day the number of children at a birth is often two, sometimes three, rarely four. The day before this was written came the report of a case of five children at a birth, all of whom seemed sound and all of whom lived. Still more direct evidence is found in the fact that occasionally in the human female there are two pairs of breasts, and very rarely three pairs. These are then disposed in a double line down the front of the body.

The new plan of caring for the young is one of the priceless heritages of the higher animals. As we rise in the grade of life the number of the young produced at one time steadily diminishes, while the care spent upon them increases. The shad may lay four hundred thousand eggs and trust them entirely to their fate. The sunfish will lay a thousand, by no means all of which can be fertilized, but it guards them somewhat after deposition. The toad lays several hundred, stores them with a considerable amount of nourishment, and protects them by a bitter deposit of mucous. The turtle has reduced the number of eggs to perhaps a score. Each of these is supplied with abundant nourishment, so that the young may develop to considerable size and activity before emerging from the egg. This material is enclosed in a firm protective shell and hidden away from sight by being buried in the ground. In the mammals comparatively few eggs are produced at one time. These are fertilized within the body of the parent, are attached to the parent, and absorb her blood. No shell is needed because nothing will kill the developing offspring that is not likely to injure the parent. Not only do the young feed upon the blood of the mother up to the time of birth, but they are practically dependent upon this same blood after birth. Though they do not take it directly from the veins, the milk is, none the less, the transformed blood of the mother. This assures the young of food as well as of protection. Best of all, the young are provided with the companionship of the mother. Now for the first time animals learn by example. Heretofore they have been born with a nearly undeviating instinct; now intelligence begins to arise. They can imitate their mother. Heretofore no acquired characters affected the young. In the mammals, although the young cannot inherit the acquired habits of the parents, they can get them by imitation, which serves nearly as well.