A first book in organic evolution

by D. Kerfoot Shute

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.

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.