338 THE POPULAR SCIENCE MONTHLY 



zoic,- but have since been in a state of degeneracy, their present repre- 

 sentatives being few in number and of a depauperate character. The 

 application of this law throughout the enormously lengthy periods re- 

 quired 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 evolu- 

 tion are steadily diminishing and will eventually cease. 



There is one direction in which paleobotany gives well-defined as- 

 surance 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. 



PALEONTOLOGY AND ISOLATION 



By Dr. JOHN M. CLARKE 



STATE MUSEUM, ALBANY, N. Y. 



THE notion of isolation as a factor in variation, as I am using the 

 term, is that of geographic separation exclusively, the concep- 

 tion 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 ex- 

 pressed in its own terms in the analysis of extinct faunas. With in- 

 creasing accuracy in the record of ancient continental lines and bar- 



