Part 12

At length the tadpole is hatched from the egg, and then soon swims about in the water (Fig. 17).

The creature that comes from the egg looks nothing whatever like a frog. It has no limbs whatever, and consists mainly of a bulky head and tail. This is the tadpole stage in the development of the frog. It can exist only in water, breathing air therefrom by means of gills. Like the fish, it has a two-chambered heart. At this stage it has no lungs, and the gills consist of an external (Fig. 17, a) and an internal pair. The mouth is small, with only horny toothless jaws, with no tongue. The creature is herbivorous, living on decaying vegetable matter. The vertebræ of the spinal column are bi-concave, as in fishes. The tadpole is essentially a fish, and would be so classed if it did not develop further. An evolving fish does not go beyond this stage. But the developing frog does go beyond this stage to a higher one. As its evolution proceeds through the multiplication and differentiation of the cells that form its body, limbs begin to bud out, first posteriorly (Fig. 17, b) and then anteriorly (Fig. 17, c). The lungs now begin to develop, and the external gills dwindle more and more until they soon disappear, the internal ones persisting for a while longer. The tongue, at this stage, also makes its appearance. The creature now can breathe both air and water. This is the permanent condition of many adult amphibians belonging to a lower order than the mature frog, such as the _siren_, _menobranchus_, etc. The siren in developing also passes through the fish stage, but does not get beyond the siren stage. But the evolving frog does go beyond this stage, for with the growth of the legs the tail dwindles slowly by its gradual absorption (Fig. 17, d). The internal gills now disappear through absorption, and the lungs develop more thoroughly. Great changes take place in the blood-vascular system, the fish-like, two-chambered heart evolving into the three-chambered, amphibian heart. In spite of its dwindling, the tail is still a very conspicuous organ. In this phase of its development the frog can breathe only air, and must frequently come to the surface of the water for that purpose, and soon leaves the water altogether. Now this stage of the creature’s development corresponds to the permanent adult condition of another order of amphibians, which is higher than that to which the siren belongs but lower than the order of the adult frog. This intermediate order has such creatures in it as the _triton_. The triton in developing passes through the fish and siren stages, but does not get higher than the triton stage. But the evolving frog goes even higher than this triton-like condition. Its tail is more and more absorbed until it finally disappears, and then the young but perfect frog appears (Fig. 17, e). During this period the teeth develop and the creature becomes carnivorous, feeding on insects. It is thus seen that the developing frog passes by small gradations from one class (the fish class) to an altogether different and higher class (the amphibian class). When it has evolved to this higher class, it then passes from the lower order (“siren” order) to a higher one (“triton” order), and then to the highest order (“frog” order). The bi-concave vertebræ of the fish-like tadpole have now developed into vertebræ with the cup-and-ball joints of the higher amphibian. It is the same with all the complex organs of the adult frog; they evolve from the much simpler structures of the tadpole.

[Illustration: FIG. 17.—Tadpoles and Frog; a, tadpole with branching external gills; b, gills absorbed and hind legs have appeared; c, fore legs have appeared; d, tail shrunk and legs enlarged; e, perfect, young frog,—tail entirely disappeared. The figures represent some stages in the life history of the frog.]

This study of the frog’s evolution from the fertilized egg is profoundly instructive. It reveals to us, through direct observation, that a creature varies in its form and structure at succeeding intervals of time. These variations diverge more and more, so that specific, generic, and even ordinal and class distinctions are revealed as the development proceeds. Owen, the distinguished comparative anatomist, in speaking of the transmutation of one species into another in the course of geologic history, says, though with a hostile purpose in view, that in the metamorphoses of the amphibians we seem to have such process carried on before our eyes to its extremest extent. Not merely is one specific form changed to another of the same genus; not merely is one generic modification of an order substituted for another, the transmutation is not even limited by passing from one order (Urodela) to another (Anura); it affects a transition from class to class. The fish becomes the frog (amphibian); the aquatic animal changes to the terrestrial one; the water-breather becomes the air-breather; an insect diet is substituted for a vegetable one. And these changes, moreover, proceed gradually, continuously, and without any interruption of active life. Such is the language of Owen in reference to these remarkable transmutations of the developing frog.

The development of the frog is a brief recapitulation, an epitome, through heredity, of the main transmutations of its ancestral forms in geologic time. It is not true that the embryonic phases in the development of a higher form always resemble the adult stages of lower forms. This may or may not be the case; but what always does occur is that the embryonic phases of a higher form resemble the corresponding phases of the lower forms. So far as the frog’s development is concerned, it is very instructive to know that the order of succession of its embryonic forms undoubtedly parallels the order of succession of corresponding forms in past geologic ages. Fishes appeared in the Upper Silurian rocks with amphibian characteristics. In the succeeding Carboniferous Ages the fishes still continued under new forms; but also the lowest forms of amphibians, the most fish-like forms, now appeared. They were somewhat like the sirens, they were perennibranchs. In the next succeeding rocks, the Permian and Triassic, higher, triton-like forms appeared. They were caducibranchs. Finally, in the Tertiary rocks, the highest forms of amphibians are found, such as the frogs.

In order to understand the relation of Ontogeny to Phylogeny, it must be carefully borne in mind that the simple and lowly organized creatures on the globe at the first appearance of life were performing the two great functions that all living creatures perform, viz.: those of nutrition and reproduction. These functions imply that organisms were reacting to environment, and, therefore, undergoing modifications and adaptations; and at the same time the organisms were giving origin to offspring—they were reproducing their kind through heredity. As these simple organisms lived through the ages and became more and more complex by modifications and adaptations to an ever-changing environment, they still evolved their kind in reproduction. Every new adaptation gained by the parent was transmitted by heredity, in the course of time, to the offspring; every form and structure modified in the parent was modified by heredity in the offspring; and every structure lost by the parent was finally lost in the offspring. Just in proportion as the parents, through the ages, became modified, often becoming more complex by the addition of adaptation to adaptation, retaining some structures of their ancestors by heredity (through use) and losing others, eventually, through disuse; so the offspring of these modifying parents became correspondingly modified, and acquired by heredity the modified structures and habits of the parents, while losing other structures in time that the parents had lost. Just as complex _organisms_ of later ages have been evolved from the simpler organisms of earlier ages by the addition of adaptation to adaptation, in an orderly sequence (Phylogeny); so, therefore, the complex _offspring_, while growing, unfold these inherited adaptations in the order of their acquisition. This last process is called Ontogeny or Embryology. Ontogeny is undoubtedly an illustration of the results of Natural Selection’s activity; for, during the phylogeny of the frog throughout the incalculable ages of the past its ancestors undoubtedly assumed innumerable forms and structures which were adaptations to the times and surroundings. But with the advancing time and changing environment, some of the old forms and structures continued useful and were retained, while others became useless and were eliminated by Natural Selection. In addition to the old useful structures that were retained changing environment often modified some of the retained structures and added still other adaptations to these. And so on, throughout the ages, in building up a frog, through geologic embryos, geologic “infants,” geologic “children,” and finally geologic adult frogs, Natural Selection has retained during ontogeny many useful structures in the order of their first appearance, and eliminated innumerable others that became useless. The ontogeny of the frog, which has been built up by its phylogeny, reveals the useful structures that have been retained, and in the order of their appearance; often showing structures that have been lost in the parent, but are not yet quite lost in the embryo, while it fails to show innumerable useless structures that have been lost in the past. This is the reason why we say that the ontogeny of a frog is a brief outline recapitulation of the main points in the phylogeny of the frog, with even some main points occasionally omitted altogether. The geologic ancestors of the frog were the scaffoldings by which it climbed from simple creatures up to its present complex organization; just as the embryological phases at present are the scaffoldings by which a simple, unicellular, fertilized ovum climbs up through heredity to the huge complexity of the multicellular adult frog. What is true of the development of the frog, ontogenetically and phylogenetically, is also true of all living creatures, and is therefore true of =man=.

Man, in his individual development, commences life as a small, microscopic cell—the fertilized ovum—which is only one-fifth of a millimeter in size. His first stage resembles an encysted protozoan animal. As cell-multiplication proceeds he soon gets into the morula stage, which resembles a colony of undifferentiated protozoans. He soon evolves into a stage which may be compared to a colony of protozoans some of the members of which have undergone differentiation. Then comes the gastrula stage, which is distinctly suggestive of a low metazoan, and in which the developing germ assumes fundamental anatomical qualities such as characterize lowly animals like polyps. Then, by gradual transmutations, the vertebrate characteristics appear; but it could not be said at this stage of development, if one did not know, whether one is observing a fish, an amphibian, a reptile, or a mammal. Finally, the developing man passes through his fish and reptile phases and reaches the mammal stage. But as yet it cannot be said to which order the animal belongs. The evolution of the individual continuing, he finally assumes those anatomical characteristics that stamp him as belonging to the order of man.

The theory of evolution, then, teaches that this development of man in the course of a few short months, like the development of the frog, is a very condensed and abbreviated epitome of the evolution of mankind from primitive protozoans during the incalculable ages of the past.

Drummond has prettily written that “the developing human embryo is like a subtle phantasmagoria, a living theater in which a weird transformation scene is being enacted and in which countless strange and uncouth characters take part. Some of these characters are well known to science, some are strangers. As the embryo unfolds, one by one these animal-actors come upon the stage, file past in phantom-like procession, throw off their drapery, and dissolve away into something else. Yet, as they vanish, each leaves behind a vital portion of itself, some original and characteristic memorial, something itself has made or won, that perhaps it alone could make or win,—a bone, a muscle, a ganglion, or a tooth,—to be the inheritance of the race. And it is only after nearly all have played their part and dedicated their gift that a human form, mysteriously compounded of all that has gone before, begins to be discerned as the resultant.”

As has been stated in the introductory part of this book, if all the animals that have ever lived on the globe should be represented by a tree those existing on the earth to-day would be indicated by the topmost twigs and leaves, while the extinct forms would be represented by the trunk and main branches. Just as the leaves, twigs, branches, and trunk of the tree have a common origin, viz., the seed that developed into the tree, so all the different species of animals of the present and the past are the trunk, branches, twigs, and leaves of the “tree of life,” and have had a common origin from a primitive protozoan cell (see Diagram of Development, Fig. 18). Therefore all creatures, living and past, have a more or less blood relationship.

The Diagram of Development will indicate in a very general way the possible track taken by a man as he evolved,—grew higher and higher as the central, straight trunk of the expanding tree of life,—during the geologic ages; and finally appeared as the inflorescence of the topmost branch of this central trunk. It is seen from this scheme that the tree of life commenced in a primitive cell. Without entering into any discussion of the various theories of evolution and epigenesis, we may say that the primitive protozoan contained potentially all the animal forms (each being a cell or group of cells) that have existed on the globe, just as the fertilized egg contains potentially all the tissues and organs (groups of cells) of the adult man.

As the tree of animal life unfolded and expanded—like a germinating seed—from the primitive protozoan, certain of the descendants evolved along the straight and central branch, through the primitive colonial protozoans, on through primitive vermes, and still on through primitive fishes (elasmobranchii), amphibians, reptiles, and on through primitive ornithodelphia (monotremes), and didelphia (marsupials) to a primitive order of monodelphia, viz.: primitive primates. The evolution of man continued through primitive anthropoidea to primitive anthropopithecus. At this point we meet with the common ancestors of the _higher anthropoid apes_ (chimpanzee and gorilla) and _man_.

[Illustration: FIG. 18.—Diagram of Development: Portion of the “Tree of Life,” showing approximately the relative places of the great groups of animals. The _Central Trunk_ and _Primary Branches_ represent _Primitive_ (geologic) forms; the _Terminal Twigs_ represent _Modern_ forms.]

At each stage of the evolution some of the descendants of the animals of this stage diverged obliquely, modifying the characters they possessed at this stage in a direction that varied more and more from those characters that led on to man. So that all along the central trunk of the tree of animal life collateral branches were given off. The collateral branches given off at each upward stage of evolution represent animals higher in the scale than those that departed from the central trunk lower down. To illustrate what has occurred at each stage in the evolution of man, pause for a moment to consider that phase of progress represented by the primitive reptilia. If we study the anatomy of the specialized reptiles, birds, and monotremes of the present, we will find that they all have many characters in common. These characters are _reptilian_. Each class has its own distinctive specialized peculiarities in addition to its common reptilian characters. The study of the fossils of the rocks shows that in the Jurassic and Cretaceous ages animals existed that were undoubtedly reptiles, but had also very distinct bird characters; also reptiles existed that had distinct monotreme characters. These reptiles came from those of earlier times that were still more generalized. As the ages passed, some of the generalized reptiles (primitive reptiles) lost more and more the reptilian features and gradually assumed more and more distinct bird characters, until finally the highly specialized modern birds (“glorified reptiles”) were evolved as a branch from primitive reptiles. The specialized reptiles of modern times likewise came from the primitive reptiles. In like manner those primitive reptiles that had mammalian (monotreme) characters, by getting into a suitable environment, gradually lost more and more their reptilian characters and assumed with increasing accentuation the characters of primitive monotremes—the lowest of the mammalian class. But observe particularly that the earliest introduced monotremes were not the specialized monotremes on the globe to-day, but generalized, primitive monotremes. These gave origin to the specialized modern monotremes, and also to the generalized primitive marsupials. The evolution of man continued through the primitive marsupials to primitive anthropoidea.

Here we meet with the common, generalized ancestors of man and the monkeys. These creatures contained, potentially at least, _anthropoid_ as well as _pithecoid_ characters. From them were derived the primitive New World monkeys (primitive Platyrrhines) and the primitive Old World monkeys (primitive Catarrhines). Some of the descendants of the primitive Old World monkeys, migrating into an environment which favored particularly the pithecoid characters, eventually developed into the tailed monkeys of the Old World (Cercopithecidæ). Others of their descendants, migrating into a different environment, found conditions that favored the anthropoid characters especially, and by greater and greater use of these, with the diminished use of the pithecoid ones, the characters of the anthropoid apes (primitive Simiidæ and primitive Simiinæ) became clearer and clearer until, in time, primitive anthropopithecus appeared,—a tailless anthropoid ape of the Old World. More than likely this anthropoid ape bore a close resemblance, as Dr. Theodore Gill long since taught, to the modern chimpanzee. If there were any differences they could scarcely have been of even a generic value. This primitive chimpanzee was undoubtedly a quadrupedal, quadrumanous creature leading an arboreal life. His descendants specialized along two distinct but closely related lines. Those that continued to live in trees specialized along the oblique path that led finally to the gorilla on the one hand and the chimpanzee on the other. Those descendants that abandoned the trees and lived on the ground used the feet more and more for purposes of locomotion and less for grasping; while they employed with increasing frequency the hands for grasping exclusively. Associated with these adaptations were many other correlated adaptations, such as the upright posture, an enlarging brain, a change in the character of the face and of the dentition, etc. As man evolved further and further along the central trunk of the tree of life, he discarded, through disuse, many of the characters that are peculiar to the anthropoid apes; and assumed with increasing emphasis, through use, many of the characters that are distinctive of man. He passed through the phases of pithecoid man and pre-palæolithic (primitive) man, until eventually, in palæolithic man, the visage of humanity is clear and unmistakable.

It is extremely interesting to attempt to form some rough picture of primitive man. It may help us to do so if we recall what Darwin has said about the Fuegians, who are among the lowest of savages. He has written that they are men whose very signs and expressions are less intelligible to us than those of the domesticated animals—men who do not possess the instinct of those animals, nor yet appear to boast of human reason, or at least of arts consequent on that reason.

The Fuegians are much nearer to the ape than to a Shakespeare or Sir Isaac Newton. In the words of Clodd, primitive man was doubtless much lower than the lowest Fuegians. “He was a powerful, cunning biped, with keen sense organs (always sharper, in virtue of constant exercise, in the savage than in the civilized man, who supplements them by science), strong instincts, uncontrolled and fitful emotions, small faculty of wonder, and nascent reasoning power; unable to forecast to-morrow or to comprehend yesterday, living from hand to mouth on the wild products of nature, clothed in skin or bark, or daubed with clay, and finding shelter in trees and caves; ignorant of the simplest arts, save to chip a stone missile, and perhaps to produce fire; strong in his need of life and vague sense of right to it and to what he could get, but slowly impelled by common perils and passions to form ties, loose and haphazard at the outset, with his kind, the power of combination with them depending on sounds, signs, and gestures.”

Through the theory of evolution it can readily be understood why the anatomical characters of the anthropoid apes and of man are so very closely alike. They have a common origin, and are blood relations—the one group of animals having specialized from common ancestors in one direction (obliquely), and the other group having specialized in another (straight) direction. (Vid. Diagram of Development.)

Man, in his individual development from a fertilized ovum, comes from a source infinitely lower than the ape. Why, therefore, should he feel such reluctance to believe that he has passed, during geologic ages, through the phase of generalized simian ancestors? Is there not much more of hope in the knowledge that he has risen higher and higher through the æons of the past than in the belief that he was created an innocent and noble character and then fell to utter wretchedness through great temptation? The motto of evolution is Excelsior. For it shows that the human race, through all the incalculable ages of the past, has risen to higher and higher levels,—to nobler and nobler phases of being. His progress in the almost infinite past suggests the hope that he will mount higher and higher towards perfection during the limitless future. Not only may we hope that there will be boundless improvement of the human race, but boundless evolution of each individual human being as well. Evolution’s motto for each individual may also be Excelsior. And, therefore, may we say with some assurance of hope that love, while kissing the pathetic lips of death, need not entertain in vain the splendid hope of immortality. For if there be no immortality of personal consciousness, then the evolution of the cosmos, of man, of the highest mind in man, have no intelligible meaning for us; they are unfathomable enigmas—idiot stories without meaning.

Man, in specializing along certain lines since separating from the ancestral simian stock, has displayed more and more that structure of his skeleton and of the soft parts molded upon it that is best adapted to the needs of the mind resident within him. His bones are not merely the jointed framework of an animal, but a framework adapted to that erect attitude which so befits his intellectual nature. His feet are not the climbing and grasping feet of the ape, but organs for giving firmness to the tread and dignity to the bearing of a creature capable of high thought. The arms and hands are not for strength alone, for these members are much stronger in many a brute; but they also give greater expression and power to the thoughts that come from within. The hands possess such molding of fingers, thumbs, and palms, such delicacy for tactile impressions, and such capacity for nice adjustments, that they are not alone used for feeding the mouth and fighting antagonists; but they also contribute pre-eminently to the desires of a large mind, and are the efficient servants of its promptings. As Dana well says, “The face, with its expressive features, is formed so as to respond not solely to the emotions of pleasure and pain, but to shades of sentiment and interacting sympathies the most varied, high as heaven and low as earth,—ay, lower, in debased human nature; the whole being, body, limbs, and head, with eyes looking, not towards the earth, but beyond an infinite horizon, is a majestic expression of the divine feature in man and of the infinitude of his aspirations.”