Page:Popular Science Monthly Volume 77.djvu/344

338 , but have since been in a state of degeneracy, their present representatives being few in number and of a depauperate character. The application of this law throughout the enormously lengthy periods required for the evolution of existing species, has led to the survival of some of the most ancient types until the present day; to the absolute obliteration of others which at one time gained great prominence; and to the gradual dying out of yet others, some of which are now found in the last stages of their existence. But through the entire course of change, the evolution of higher and yet higher forms has been the most conspicuous fact. Furthermore, it is undoubtedly true that the general course of evolution is in progress to-day as in the past, since all the potentialities of such evolution exist now as always, though conditioned by the fact that owing to continued changes in the physical character of the earth's atmosphere as well as of its crust, the possibilities of evolution are steadily diminishing and will eventually cease.

There is one direction in which paleobotany gives well-defined assurance that the evidence derived from existing species leads to correct conclusions. In tracing the succession of types, we are led to the belief that there is no direct sequence. Conterminous evolution is in accord with neither theory nor ascertained facts, and it is, therefore, impossible to conceive of a figure which shall in any way represent a single and unbroken line of succession. If paleontology teaches us anything, it is that each great phylum, as well as its various subdivisions, finally reaches its culmination in a terminal member from which no further evolution is possible. But that from some inferior member, possessing high potentialities, a side line of development arises. There is thus, in the early life of each member of the series, a certain recapitulation of ancestral characters. This conception of a continuance of the main line of descent through a succession of lateral members is both logical and fully in accord with the evidence derived from both recent and extinct forms of plant life, as well as with our present theory of evolution.



HE notion of isolation as a factor in variation, as I am using the term, is that of geographic separation exclusively, the conception expressed most clearly by Wallace, Moritz Wagner and Jordan. I take it that while this influence has been carefully estimated in the geographical distribution of living species, it has not often been expressed in its own terms in the analysis of extinct faunas. With increasing accuracy in the record of ancient continental lines and 