First, passing by that long series of geological formations in which no remains of life have been found, or only those of boneless or invertebrate animals, we find ourselves in a sea abounding in stone-lilies and huge crustaceans,[190] having among them the small forms of the earliest fish known to us, those having gristly skeletons. Then as the scene passes on, and forests clothe the land, we behold the descendants of these small fish becoming large and important, wearing heavy enamelled plates or sharp defensive spines; some of them with enormous jaws, two or three feet in length, wandering in the swamps and muddy water, and using their air-bladder as a lung. But these did not turn their air-breathing discovery to account; they remained in the water, and their descendants are fish down to the present day. It is in the next scene, when already the age of the huge extinct fishes is beginning to pass away, and tree ferns and coal forest plants are flourishing luxuriantly, that we find the first land animals,[191] which have been growing up side by side with the fish, and gradually learning to undergo a change, marvellous indeed, yet similar to one which goes on under our eyes each year in every country pond. For now, mingling with the fish, we behold an altogether new type of creatures which, beginning life as water-breathers, learn to come out upon the land and live as air-breathers in the swamps of the coal forests. A marvellous change this is, as we can judge by watching our common tadpole, and seeing how during its youth its whole breathing organs are remade on a totally different principle, its heart is remodelled from an organ of two chambers into one of three, the whole course of its blood is altered, some channels being destroyed and others multiplied and enlarged, a sucking mouth is converted into a gaping bony jaw, and legs with all their bones and joints are produced where none were before, while the fish’s tail, its office abandoned, is gradually absorbed and lost. The only reason why this completely new creation, taking place in one and the same animal, does not fill us with wonder is, that it goes on in the water where generally we do not see it, and because the most wonderful changes are worked out _inside_ the tadpole, and are only understood by physiologists. But in truth the real alteration in bodily structure is much greater than if a seal could be changed into a monkey. Now this complete development which the tadpole goes through in one summer is, after all, but a rapid repetition, as it were, of that slow and gradual development which must have taken place in past ages, when water-breathing animals first became adapted to air-breathing. Any one, therefore, who will take the spawn of a frog from a pond, and watch it through all its stages, may rehearse for himself that marvellous chapter in the history of the growth and development of higher life.