Page:Popular Science Monthly Volume 12.djvu/199

Rh existing ones. In the Secondary, reptiles, and out of them birds, were developed; the decreasing amount of carbonic acid and the increasing amount of oxygen permitting that change. Of birds, the earliest had a long, lizard-like tail, composed of thin vertebrae, to every one of which were attached strong, rudder-like feathers in pairs. The same formation of the tail part of the vertebral column still occurs transiently in the embryos of later birds. The transition from the reptile to the bird is manifested by some of the latter having teeth set in one order in grooves, in another in distinct sockets. Among mammals as among fishes the imperfect appeared first. About the middle of the Mesolithic period, out of a branch of the cloacal animals the marsupials were evolved; and in the beginning of the Tertiary the placentals were developed out of the marsupials. The latter were at one time distributed over the whole earth; now they are fast approaching extinction. In Europe, Asia, Africa, not a single member of the group remains. The cloacal animals, the marsupials, the placentals, stand therefore in an order of succession.

Such has been the order of evolution in Europe. For its order in America I may refer you to the recent admirable address of Prof. Marsh before the American Association for the Advancement of Science. The general conclusions at which we arrive in one case are substantiated in the other.

In accordance with his descent, the cloacal structure exists in man in the earlier period of his embryonic life. The separation into two openings takes place about the twelfth week of his uterine development. Shall we not, therefore, infer from the evidence of his embryonic forms that he has been developed step by step out of the lower vertebrates?

In the early stages of their evolution, amphibia, reptiles, birds, cannot be distinguished. The first steps of development in all vertebrates are identical. Man passes now through the same series of transmutations which his animal predecessors passed through in immense spaces of time, long ago. The progress he makes in the lapse of a few days in the darkness of the womb is the same that has been followed by the procession of animated Nature in the lapse of myriads of centuries in the daylight of the world.

From a comparison of their studies embryologists and paleontologists unite in the conclusion that individual development is a rapid repetition of race-development, and that the paleontological movement is to be interpreted by the embryonic. The connecting links supposed to be missing in the former may be sought for in the latter. Individual development, paleontological development, and comparative anatomy, through their combined evidence guide us to a deduction of the genealogy of any organism. The dominion of law is everywhere manifest. The capricious intrusion of a supernatural agency has never yet occurred.