The Meaning of Evolution Part 7

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 s.p.a.ce 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 a.s.similated 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 furnis.h.i.+ng air to the chick.

At the same point at which the blood vessels from the yolk enter the chick, another set of vessels pa.s.s in and out. These are attached to a large flattened bag which floats above the embryo against the upper side of the sh.e.l.l. This bag is called the allantois, and serves as a sort of lung for the developing chick. The sh.e.l.l is porous enough to allow air to pa.s.s through it. The blood vessels of the allantois take in oxygen and give out carbon dioxide through the porous sh.e.l.l. 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 sh.e.l.l 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 sh.e.l.l 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 sh.e.l.l 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 b.r.e.a.s.t.s. 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.

These creatures are the lowliest of the cla.s.s 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 pa.s.ses 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 sh.e.l.l of the egg and absorbs oxygen through the sh.e.l.l. Inside the body of the mammal this is impossible, because the air is too far away. No sh.e.l.l 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, b.r.e.a.s.t.s.

It would seem as if here we had an entirely new organ. No other animal gives nourishment to its young in such fas.h.i.+on; 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 a.s.sist 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 pa.s.ses 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 acc.u.mulate, 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 b.r.e.a.s.t.s 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 was.h.i.+ng, 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 const.i.tuent. 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 s.p.a.ce 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 b.r.e.a.s.t.s 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 diminis.h.i.+ng number of young and greater care of them as we rise in the scale has come also a diminis.h.i.+ng number of b.r.e.a.s.t.s 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 b.r.e.a.s.t.s 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 b.r.e.a.s.t.s, 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 sh.e.l.l 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 sh.e.l.l 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 a.s.sures the young of food as well as of protection. Best of all, the young are provided with the companions.h.i.+p 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.

There is, however, a more wonderful advantage that comes from the close attachment between mother and offspring. This intimate relations.h.i.+p brings about an affection of the mother for her young heretofore unknown in the animal world. It is somewhat paralleled among birds, but here the care of the nestling is less intimate, far less maternal, than the care of the mammal for her young. As the number of the young grows less and the care taken of them increases, the intensity of the affection also increases. By the time we get as high as the dog or the cat this fondness becomes a fierce, self-sacrificing love. When we come to man, with his high intellectual powers, with his deeper moral sense, we find a wonderful change. This love of the mother for her child has grown into the finest emotion possible to the human heart. It no longer is confined to the dependent life of the child, but follows the offspring through its entire life, guiding, guarding, shaping its destiny, handing on to the child the treasured wisdom of the race. Influenced by the example of the mother, the father comes to have a love for his children. It is not so strong as that of the mother, nor so utterly unselfish, but it is still a n.o.ble and exquisite love. Developing in a different direction, the love of the mother for her children grows as civilization advances, and spreads over the father of those children as well. Again reflecting her love, the man finds himself filled with a new feeling for the woman. It is never as unselfish, as free from desire, as is her love, but it completely transforms his relation to her. What has been with him simply desire is enn.o.bled and enriched until it becomes the finest pa.s.sion of his life, absolutely transforming him, in relation to her, from a selfish brute into a tender and life-long companion. So utterly does the love thus engendered transfigure human life that when we seek to express the divine nature in human terms, and these are the only terms we know how to use, the richest revelation that has come to us is the conception taught by the Master that "G.o.d is Love" and that "as a father pitieth his children, so the Lord loveth them that fear him."

CHAPTER VIII

THE STORY OF THE HORSE

Ever since men have been familiar with the idea of evolution there has been a temptation on the part of the zoologist to draw up pedigrees expressing the relations.h.i.+p between the various groups of the animal kingdom. The impulse is natural, and, if the resulting tables are not accepted with too much confidence, the result is not undesirable. The truth of the matter is that all of these pedigrees are more or less hypothetical. They simply show what connection seems most likely. In all of them are s.p.a.ces filled with doubtful names. Each addition to our acquaintance with the past history of animals necessitates revision of our tables. The student of fossils, trying to rebuild in imagination the world of the past, finds himself often strangely unable to link these animals together. The result is that the more we know of fossils, the more distrustful we become of the easy connections we have been making between groups. Accordingly we are more than commonly pleased when we find the clear indication of a genuine pedigree, actually ill.u.s.trated by real examples, following each other in time through the geological history. A few of these lines are gradually becoming plain, and none of them is clearer than the pedigree of our familiar and much loved horse. The example is a particularly interesting one, not only because of our affection for the animal, but because the horse originated in all likelihood in North America on the land occupied to-day by our Western plains. As though he loved the country of his ancestors, he returned after having circled the globe, and once more went wild in the home of his forefathers. The problem was first worked out in Europe and later elaborated in this country. Now the history gets its finest expression in the American Museum of Natural History in New York City. The collection of fossil horses in that inst.i.tution surpa.s.ses in completeness and in excellence of mounting and of sympathetic restoration any similar collection representing the ancestry of any other animal in the world.

In the table of Geological Times, given in chapter six, the era of recent life known as the Cenozoic is seen to occupy something like five million years. This figure, as was previously suggested, is very uncertain, and may be three or may be six, but is safely represented in millions. Through most of this time stretches what is known as the Age of Mammals, the Tertiary Age. Its close, occupying only the last few hundred thousand years, is known as the Age of Man, the Quaternary. Through perhaps three or four millions of these years stretches the known pedigree of the horse.

When we go back to the early Tertiary we find a forest, with trees that shed their leaves, interspersed with glades, in which already the gra.s.ses were beginning to be developed. This state of affairs had existed but for a comparatively short time, geologically speaking. It had come only in the latter part of the preceding era. Lake and swamp, meadow and forest intermingled to make a rich and varied scene. Slowly the land toward the western side of North America lifted itself into plateau and mountain range. Slowly the westerly winds began to be cut off by the barriers thus raised across their path. As they swept over the plateau and down into the eastern plain their moisture came to be diminished. Gradually a very different state of affairs set in. The ground became harder, the forest became spa.r.s.er, the plants became higher and firmer, the gra.s.ses tougher and more wiry, and, by the time the Quaternary arrived, a condition probably even drier than that of to-day existed over our western highlands. Throughout this long change, spread over millions of years, a creature which has become our horse steadily persisted and steadily advanced. Side lines developed which finally disappeared, but the main line kept on, and when the Quaternary came the horse arrived with it. Many of the skeletons in this series were known before it was realized what they were. As time went on and intermediate forms were found, it became possible to recognize these as ancestors of the horse and to a.s.sign them their proper position in the family tree.

[Ill.u.s.tration: THE EVOLUTION OF THE HORSE'S FOOT

_After H. F. Osborne and Charles R. Knight. By permission of the American Museum of Natural History._]

The earliest of the forerunners of the horse with which we are acquainted would certainly not be recognized as such by any but the most careful student of animals, if we could see him to-day. He stood not higher than a fox-terrier dog, though his shape was very different. But he would probably be more likely to be cla.s.sed with the dog than with the horse by the hasty observer, for he walked with four toes of each foot upon the ground as the dog does to-day. Like the dog, he had hanging at the inner side of his front foot a little useless toe. He was long in body, comparatively short of leg, a little long of head and neck, and distinctly long of tail. His grinding teeth had points on them not unlike a pig's, and possessed no apparent resemblance to the wonderful curved and ridged surfaces seen on the teeth of the modern horse. What his skin and hair were like can only be conjectured. In the restoration which Mr. Knight has made, at the suggestion of Professor Osborne, an interesting inference has been drawn. That he was a creature of the forest is suggested by his spreading toes, which would keep him from sinking in the soft soil. It is consequently surmised that he was dappled with spots which allowed him to rest unnoticed on the sun-flecked floor of the forest. Mane he had none, and his tail was probably tufted slightly at the end with hairs, which were increasingly short as they approached the top. He had no forelock, and the hair along the ridge of his neck was a little longer than the rest, and stood erect. Browsing about on the soft and tender herbage of his woodland home, his teeth had as yet no tendency to become specialized. The molars had mounds upon them, developing, perhaps, more into the shape of the points of the hog's, but even still quite generalized teeth. His main enemies, from whom, perhaps, he could with little difficulty escape, were creatures related to the hyenas of to-day. Perhaps, like their modern representatives, they preferred eating their flesh tainted to exerting themselves enough to capture and kill their prey. By the time we advance a little further into the Tertiary, though still in its early portion, a remarkable change has already come about. The fifth toe, which in the earliest horse hung upon the side of the front foot, has completely disappeared. The change in the hind foot has gone still further. The hind leg in many animals evolves more rapidly than the front. The heavy work of running is always done by the hind feet, while the front feet serve rather as a prop to keep the animal from falling than as the actual means of locomotion. Hence the hind feet and the muscles of the hind quarters are almost always heavier than the front. Possibly on the front foot the little fifth toe was less of an obstruction, and persisted after the early horse had lost the corresponding toe on his hind foot. This process has gone on still further in this second stage, and the hind foot has but three toes, while the front still has four. This is not the only advance. Already the middle toe of the original set of five is becoming emphasized. The weight is thrown more forcibly upon it, as with the human foot it is upon the inner or big toe. The middle toe is growing larger and larger, and the nail upon it is spreading around it and is growing firmer. The creature, too, is standing more nearly upon his toes; his legs are getting longer; he stands higher from the ground, and now has come to be the size of a hound.

We can only surmise why this creature should have undergone such a change, but the presence of flesh-eating animals having the size of a fox, and presumably of the fox's swiftness, probably tells the story.

The little bands of early horses, pursued by their carnivorous foes, were slowly modified into swifter creatures. It is not so much that running made them fast, as that the slow ones were continually being caught. If this process of constant elimination of the slow members of any herd is kept up long enough, the group will necessarily develop speed. As time goes on, of these early horses those which happened to have longer legs and stood higher upon their toes won in the race, and handed on their qualities to their long-legged descendants. As the animal rose upon his toes, the inner toe, corresponding to our thumb, was first raised off the ground and rendered useless, while a similar change came over the corresponding toe on the hind foot. The hard work of running being done on the latter, this superfluous toe was more detrimental there than on the front foot, and disappeared, consequently, more rapidly. In time, however, it also disappeared from the front foot. Gradually the further elevation of the foot lifted the toe, which corresponds to our little finger, off the ground, and this now disappears also.

With increasing toughness of the gra.s.ses, as the climate becomes drier and the region more elevated, the teeth of the horse are given harder work. The points begin to spread into ridges and to unite with each other in such way as to form the crescents, which are later to be so characteristic of the teeth of the modern horse.

By the middle of the Tertiary this ancestral horse has risen in height until he is taller and heavier than a setter dog. Three toes are found on each front foot. The middle toe is getting constantly more developed, though the smaller toes are evidently still of use. The ridges of the teeth are quite crescentic now on the outer side, and becoming better adapted to the evidently firmer food which the creature is obliged to eat.

As we come toward the end of the Tertiary, the development which had been all pointing in one direction has advanced very much further. The creature now would be undoubtedly recognized by anyone as a horse. The legs are longer and straighter; the middle toe has become the only useful toe, though on each foot a smaller toe, slender and probably useless, still hangs on either side. Two similar useless toes to-day hang at the back of the foot of the cow, which is now walking upon her two toes, which give her the appearance of carrying a cloven hoof.

That is to say, the first toe on the foot of the cow has disappeared, the second and fifth hang useless and much diminished at the back of the foot, while the third and fourth are both well developed and serviceable in walking.

The late Tertiary horse has grown to be the size of a burro of to-day, though probably it was a little more slender. The teeth are quite horselike, both in shape of the crescentic ridges on their surface, in the length of the teeth in the jaw bone, and in the fact that the crinkled edges of enamel on the upper surface are protected on either side by dentine or by cement. These surfaces, being softer than the enamel, wore away somewhat more rapidly and allowed the sharp edges of enamel to stand up in ridges. This plan increases the grinding power of the teeth.

With the oncoming of the Era of Man the horse reaches his modern splendid development. During the early Quaternary the horse was perhaps in some of his representatives a larger creature than he is to-day. Each foot now has but a single toe. The nail has spread around firmly and heavily, until it has become a splendidly developed hoof, permitting the animal to travel with speed over firm and often stony ground. The side toes have disappeared completely from the outside of the horse's leg, although upon removing the skin it is easy to find the long splints, which are the remnants of toes, which have not yet quite disappeared. His heel has been lifted in the air until it is eighteen inches off the ground, and he is standing like an expert dancer upon the tip of his toe. The body of the horse thus being lifted far off the ground, a new development becomes necessary. All through the growth of the creature the neck and head have been obliged to lengthen correspondingly. Every animal must be able to bring its head down to the level of its feet in order that it may drink. Various animals use different methods to accomplish this result. The giraffe, with his enormously long legs, has a correspondingly long neck, which lowers his mouth to the ground. Even with this extended neck the giraffe's legs are so exceedingly long that he is obliged to spread his front feet when he wishes to reach the ground with his head. The elephant has pursued exactly the reverse plan. Using his tremendous head as a battering ram in fighting, and using his enormous tusks both in battle and in uprooting young trees, a lengthened neck is absolutely out of the question. Furthermore his front teeth have grown so prodigiously that they would interfere with his getting his mouth to water. Accordingly, his nose has lengthened its tip until it reaches the level of his feet, and this nose becomes to him the main organ of grasp and of touch. To drink, its end is inserted in the pool and water is drawn up the nostril. If the animal were to attempt to draw it all the way back into his throat, it would inevitably strangle him by getting into his windpipe. Accordingly, when the nose is well filled with water, the tip of it is inserted in his mouth, and the water discharged by a quick puff. The horse has taken a method intermediate between these. It had moderately lengthened both neck and head in order to get to the ground with its nipping teeth, and thus to gather the gra.s.ses which serve as its princ.i.p.al food.

The mammalian teeth, while of four kinds, really in most animals serve but two purposes. The front teeth consist of the incisors and canines, and are used for biting. The hind teeth, consisting of premolars and molars, are used for grinding. In the horse, the jaw has lengthened between these two sets, carrying the biting teeth far forward of the molars. It is this gap in the row of the horse's teeth which makes it possible for us to insert the bit into his mouth.

Now comes a strange accident into the life of our American horse.

Creatures of the same kin had been evolving in Europe and Africa, but the developments are more distinctly horselike, it would seem, in our own country. Then for some reason the horse disappeared completely from American soil. Doubtless two things happened. First of all, some of them migrated across a stretch of open country which then connected America with Asia in the neighborhood of Bering Strait. These creatures spread first over Asia and then over Africa and Europe, leaving their skeletons scattered over this enormous stretch of country. a.s.ses and zebras are still found abundantly and widely scattered, but the wild horse of to-day is seen only in western Asia.

What happened to those who remained in America we shall possibly never know. Some surmise that a fly not unlike the tsetse-fly of Africa killed them out. Perhaps the members of the cat family, which are steadily growing larger and fiercer, fed on their young if not upon the older ones, and exterminated them. Perhaps the Glacial period which followed was too cold for them. But, whatever may have been the cause these horses died out not only in North but also in South America, to which country they had spread.

The old world horse was the companion of man. The skeletons of those found with early man in the caves of Europe look as if the horse had been a creature to draw man's burdens and to serve him for food, rather than to bear him upon its back. Its roasted bones are often found about the old tribal fires. Upon the discovery of the new world the Spaniards brought with them to Mexico and to the Mississippi Valley the horses which carried them in their battles against the Indians. In the course of these frays many riders were killed and their horses roamed wild. Slowly they made their way to the western plains; gradually they became tougher and more wiry; their diminished hoofs learned to catch more carefully in the rocks of their mountain home; and the mustang and bronco of more recent years are the descendants of the little dawn horse, whose dainty skeleton is found in the rocks over which his later descendants, after a long stretch of perhaps four million years, are now running.

CHAPTER IX

EVOLUTIONARY THEORIES SINCE DARWIN

In considering the value of Charles Darwin's work and its permanent effect upon the thought of mankind, we must be careful to distinguish between two phases of his effort. It was his aim to prove two propositions: first, that there is such a process as evolution; second, that he had discovered the method by which evolution is accomplished. Before his time there was no general agreement as to the fact of evolution. People generally thought the idea absurd, as well as irreligious. All previous efforts on the part of advanced thinkers to persuade mankind of the truth of evolution had been nearly without effect. Among the early philosophers the whole idea was purely speculative. They made no attempt to prove it, and the conception was without influence upon the thinking of the ordinary man. This remains true until the time of Lamarck. This French genius succeeded in persuading not a few people of the validity of the idea of evolution.

He probably could have convinced many more had it not been for the hostility of Cuvier. Accordingly, Charles Darwin's "Origin of Species" fell upon a world entirely hostile to the idea, when it thought of it at all. Within fifty years of the publication of this wonderful book, probably the entire scientific world is agreed that evolution, in some form or other, is the undoubted solution of the mystery of creation. The materialist may think of it as a mechanical process relentlessly working itself out without design or purpose. The theist will accept it as the plan by which Eternal Power steadily works. The devout Christian or Jew will see in it G.o.d's method of creation. The idea of development has penetrated every science that has to do with animals or man. It is even beginning to influence such inorganic sciences as Physics and Chemistry. We now hear of the evolution of the elements, and the evolution of forces. The world has been persuaded that evolution is true, and this is primarily the result of the work of Charles Darwin. It is astonis.h.i.+ng that so great a revolution should have come in so short a time.

The other phase of Darwin's work was his attempt to find the agent which is bringing about the actual transformation of animals and plants. As we have seen in the preceding chapters, it was his idea that natural selection was the efficient agent which constantly eliminated all unfit variations, leaving only the best to carry on the work of the world and to reproduce their own fit kind. Many biologists since his time have doubted whether unaided Natural Selection will account for the constant advance in organisms. This is the part of the work which is often seriously questioned.