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.