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.
Only a few of the countless details have been mentioned which demonstrate the resemblance of the successive stages first to fishes, and later to amphibia and reptiles. We have a wide choice of materials, but even the foregoing brief list of illustrations shows that the order in which the stages follow is the one which comparative anatomy independently proves to be the order of the evolution of fishes, amphibia, reptiles, and birds. Why, now, should it be necessary for a developing bird to follow this order? The answer has been found in the immense array of embryological facts that investigators have verified and classified, that all tell the same story.
It is, that birds have arisen by evolution from ancestors which were really as simple as the members of these lower classes. It seems then that the only way a bird of to-day can become itself is to traverse the path along which its progenitors had progressed in evolution. Stating its conclusions precisely, science formulates the principle in the following words: _individual development is a brief résumé of the history of the species in past times_, or, more technically, _ontogeny recapitulates phylogeny_. To be sure, the full history is not reviewed in detail, for the chick embryo does not actually swim in water and breathe by means of gills. Only a condensed account of evolution of its kind is presented by an embryo during its development; as Huxley and Haeckel have put it, whole lines and paragraphs and even pages are left out; many false passages of a later date are inserted as the result of peculiar larval and embryonic needs and adjustments. But in its major statements and as a general outline, the account is a trustworthy natural document submitted as evidence that higher species of to-day have evolved from ancestors which must have been like some of the present lower animals.
Coming now to the mammalia, it might seem that we have reached forms so highly developed that they would not exhibit the same kind of developmental history, but would have their own mode of growing up. This is not so, for like the adult fish, the larval tadpole, and the embryo chick, an embryo of a cat or a man is at one time constructed with a series of gill-clefts and with blood-vessels and skeletal supports of fishlike nature that are everywhere associated with gills. The embryos of wildcats and dogs, rabbits and rats, pigs, deer, and sheep, and of all other mammalia, possess similar structures.
Thus they all pass through a stage which is found also in the development of reptiles, birds, and amphibia,--a stage which corresponds to the fish throughout its life. Unless these facts mean that the great classes of vertebrates have originated together from the same or closely similar ancestors, they are unintelligible; for we cannot see why a cat or a chick should have to be essentially fishlike at any time unless this is so. Comparative anatomy states as we have learned that the amphibia as a class have evolved from and have out-developed the fishes, that reptiles have progressed still higher, and that birds and mammals have originated from reptilian ancestors along roads that have diverged beyond the immediate parent class. Because the members of each class have to pass along the same path trodden by their many varied ancestors, although at express speed, as it were, the similarity of the earliest stages in their development is explained, for during these periods they are traversing a path over which their ancestors passed together.
The places where the developing embryos depart from the common mode show where the several divisions took leave of one another in their evolution,--a point that comes out with great clearness when the facts of mammalian development are broadly compared. The embryos of carnivora and rodents and hoofed animals are alike in their earlier development, and their agreement means a community of origin. At a certain point the cat and dog depart from the common mode, but they remain alike up to a far later stage than the one in which they are similar to the embryos of rats and sheep.
The rat and squirrel and rabbit, on their part, remain together until long after they take leave of the carnivora and ungulates; while the sheep and cattle and pigs have their own branch line, which they follow in company after leaving the embryos of the other orders. The reasons for these facts seem to be that the members of the three orders exemplified have evolved from the same stock, which accounts for their embryonic similarity for a long time after they collectively come to differ from amphibia and reptiles, while the members in each order became differentiated only later, wherefore their embryonic paths coincide for a longer period.
Thus the degree of adult resemblance which indicates the closeness of relationship corresponds with the degree of embryonic agreement; that is, the cat and dog are much alike and their modes of development are essentially the same to the latest stages, while the cat and horse agree only during the earliest and middle stages, and their lines diverge before those of the cat and dog on the one hand, or those of the horse and pig on the other.