The Doctrine of Evolution: Its Basis and Its Scope

by Henry Edward Crampton

I have said that development is a "natural" process. We employ this word for the familiar and everyday occurrence or thing; it does not imply that everything is known about the object or phenomenon, because science knows that complete and final knowledge is impossible. We say that it is natural for rain to fall to the earth, and we speak of the law of gravitation according to which this takes place as a natural principle, but it may not have occurred to many to inquire what makes rain fall and why do masses of matter everywhere behave toward one another in the consistent manner described by the law in question. Sunshine is natural, but we do not know why light travels as it does from the sun to the earth, and this is another question which, like the inquiry into the ultimate cause of the familiar and natural phenomenon of gravitation, has not yet been answered. But it is still regarded as natural for the rain to fall and for the sun to shine. In the same way does science view development, denoting it natural because it is an ordinary everyday matter. And we are under no more obligation to postulate supernatural control for the changing forms in the life-history of a chick or a cat than we need to assume that gravitation and the radiation of light demand immediate supernatural direction. The embryology of no form is fully understood or described or explained, but no intelligent person would be willing to assert that because complete knowledge is lacking, it is unnatural for organic transformation to take place during growth. Whatever may be the ultimate origin and nature of the directing powers behind gravitation and development and other phenomena, we have no concern with such matters because they cannot be handled by scientific methods and one belief about them is on the same plane with any other. Our task is to deal with the everyday phenomena of life and the production of living species.

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It is not necessary to go far afield to find an animal which will introduce us to the general principles of embryology. In the present instance as in the case of comparative anatomy almost any form will disclose the meaning of development, for animate nature is uniform and consistent in its methods of operation throughout its wide range. We shall begin with the familiar frog which every one knows is a product of a tadpole; passing on to the chick we will learn more facts that will enable us to formulate the main principle of comparative embryology in definite terms; we will then be prepared to extend our survey so as to include somewhat less familiar facts and animals that are even more significant than the first illustrations.

If we should visit a woodland pond in early spring, we would find somewhere among the leaves and sticks in the water large masses of a clear jellylike consistency enclosing hundreds of little black spheres about an eighth of an inch in diameter. These are the egg masses and eggs of a common frog. Watching them day by day we see the small one-celled egg spheres divide into more and more numerous portions which are the daughter-cells, destined to form by their products the many varied tissues and organs of the developing larva and adult frog. After three or four days the egg changes from its globular form into an oval or elliptical mass, and from one end of this a small knob projects to become a flattened waving tail a few days later. On the sides of the larger anterior portion shallow grooves make their appearance and soon break through from the throat or pharynx to the exterior as gill-slits. Shortly afterwards the little embryo wriggles out of its encasing coat of jelly, develops a mouth, and begins its independent existence as a small tadpole, with eyes, nasal and auditory organs, and all other parts that are necessary for a free life. Thus the one-celled egg has transformed into something that it was not at first, and in doing this it has proved the possibility and the reality of organic reconstruction.

The tadpole breathes by means of its gills, and it is at first entirely devoid of the lungs which the adult frog possesses and uses. When we speak of the larval respiratory organs as gills we imply that they are like the organs of a fish which have the same name; they are truly like those of fishes, for the blood-vessels which go to them are essentially the same as in the lower types and they are supported by simple skeletal rods like the gill-bars of the fish. In a word, they are the same things.

The animal feeds and grows during the months of its first summer, and hibernates the following winter; with the warmth of spring it revives and proceeds further along the course of its development. Near the base of the tail two minute legs grow out from the hinder part of the body, and while these are enlarging two front legs make their appearance a little behind the gills. The tadpole now rises more frequently to the surface where it takes small mouthfuls of air. Meanwhile great changes are effected inside the body where the various systems of fishlike organs become remodeled into amphibian structures. A sac is formed from the wall of the esophagus, and this enlarges and divides to form the two simple lungs. The legs increase in size, the tail dwindles more and more, the gills close up, and soon the animal hops out on land as a complete young frog. From this time on it breathes by means of its lungs instead of gills, even though it returns to the water to escape its foes, to seek its prey, and to hibernate in the mud of the lake bed during the winter months.

All these changes are familiar and natural, but until science places them and similar facts in their proper relations their significance is lost to us. The tadpole is essentially a fish in its general structure and mode of life, even though its heritage is such that it can develop into a higher animal. When it does become a frog it proves beyond a doubt that there is no impassable barrier between fishes and amphibia. Our earlier comparison of the structures of these two classes of vertebrates led to the conclusion that the latter had evolved from antecedents like the former, and had thus followed them upon the earth; now that sequence seems to have some connection with the method by which a tadpole, obviously not a fish but nevertheless actually fishlike, changes into a frog, a member of a higher class of vertebrates. This method is employed by developing frogs apparently because it follows the ancestral order of events, and because, so to speak, the only way a frog knows how to become a frog is to develop from an egg first into a fishlike tadpole and then to alter itself as its ancestors did during their evolution in the past. We begin to see, then, that in addition to the impressive fact of development itself, the mode of organic transformation is far more conclusive evidence of evolution, because it reveals an order of events which parallels the order established by comparative anatomy as the evolutionary sequence.

However it is well to review some of the changes by which a chick comes into existence before attempting to comprehend fully the fundamental principle of development that the tadpole's history discloses to us. The egg of a common fowl is certainly not a chick. Within the calcareous shell are two delicate membranes that enclose the white or albumen; within this, swung by two thickened cords of the albumen, is the yellow yolk ball enclosed by a proper membrane of its own. In the earliest condition, even before the albumen and the shell are added and before the egg is laid, on one side of the yolk-mass there is a tiny protoplasmic spot which is at first a single cell and nothing more. The hen's egg is relatively enormous, but nevertheless, like that of the frog, it starts upon its course of development as a single unitary biological element—a cell. During the earliest subsequent hours the first cell divides again and again to form a small disk upon the surface of the yolk. Soon the cells along the middle line of this small sheet become rearranged to make an obvious streak or band, and about this line a simple tube is constructed which is destined to become the future brain and spinal cord. The whole disk continues to enlarge by further division of its constituent elements so that it encloses more and more of the yolk mass, but the little chick itself is made out of the cells along the central line of the original plate, from which it folds at the sides and in front and behind so as to lie somewhat above and apart from the flatter enclosing cell layers which partly surround the yolk.

At the sides of the primitive nerve-tube small blocks of cells arise to develop into primitive muscles and other structures. As nourishment is brought to the embryo from the surrounding layers enclosing the nutrient yolk, one system after another takes its shape and builds its several parts into organs which can be recognized as elementary structures of a chick. Among the more interesting ones are small clefts or slits formed in the side walls of the rudimentary throat or pharynx. Blood-vessels go forward from the simple heart to run up through the intervening bars exactly as in the tadpole and the fish. In brief, the young chick possesses a series of gill-slits, for these structures are the same in essential plan and relations as the clefts of tadpoles and fishes. Does this mean that even birds have descended from gill-breathing ancestors? Science answers in the affirmative, because evolution gives the only reasonable explanation of such facts as these. The case seems different from that of the frog, because gills are used by the tadpole, but gill-slits and gill-bars can have no conceivable value for the chick as organs concerned with the purification of the blood. None the less, if the transition from a gilled tadpole to the adult with lungs means an evolution of amphibia from fishlike ancestors, then the change of a chick embryo with gill-clefts into the fledgling without them is most reasonably interpreted as proof that birds as well as amphibia have had ancestors as simple as fishes.

As development progresses four small pads make their appearance; two of these lie on either side of the body back of the head and the other two arise near the posterior end. They are far from being wings and legs, but as day follows day they become molded into somewhat similar limbs, as much alike in general plan as the four legs of a lizard; subsequently the ones at the front change into real wings and the hinder ones become legs. Meanwhile the internal organs slowly transform from fishlike structures into things that display the characteristics of reptilian counterparts, and only later do they become truly avian. Last of all the finishing touches are made, and the whole creature becomes a particular kind of a bird which picks its way out of the shell and shifts for itself as a chick.