306 THE EVOLUTION THEORY 



processes of selection, and to represent the whole of evolution as 

 a process conditioned by this phyletic force, which would have given 

 rise to the world of organisms which has actually arisen, even if the 

 conditions of life at the different periods of the earth's history had 

 been other than they were. I have always combated this idea, w^ith- 

 out however overlooking that it is based upon facts which — at that 

 time at any rate — gave it a certain justification. We cannot pass 

 it by without giving some other interpretation of the facts. Follow- 

 ing Nageli, the botanist Askenasy championed this view of ' variation 

 in a different direction,' which gives rise to new forms ; and in more 

 recent times Romanes, Henslow, and Eimer expressed similar views, 

 and — although they did not actually dispute the existence of pro- 

 cesses of selection — they attributed a much less important role to 

 them, and referred the phyletic genealogical tree of organisms in the 

 main to other and internal causes. ' 



Like Nageli himself, his followers have laid stress upon the 

 fact that natural selection cannot be the cause of the evolution and 

 succession of particular species, because the differences which separate 

 species from species are not of an adaptive nature, and therefore 

 cannot depend upon selection ; but if the step from one species to the 

 next succeeding one does not depend upon adaptation, then the 

 greater steps to genera, families, and orders cannot be referred to it 

 either, since these can only be thought of as depending upon a long- 

 continued splitting up of species. Genera, families, and all higher 

 groups must be recognized as conventional categories, not as real 

 divisions existing in nature itself. Even Treviranus and Lamarck 

 maintained that the differences between genera depended just as 

 much upon our estimate, our intellectual convenience, as do the 

 differences between species. All forms were originally connected, 

 though they may not be so now, and if the species are really not 

 distinguished by adaptive characters, then neither are any other 

 grades of our classificatory system, neither order nor classes, since 

 they all depend originally on the transmutation of species. It was 

 therefore quite consistent of Nageli to seek the mainspring of organic 

 evolution, not in adaptation, but in an unknown evolutionary force. Thus 

 he refused to recognize adaptation as a consequence of selection, but re- 

 garded it, as Lamarck had done, as the direct effect of external conditions, 

 and as an entirely subordinate factor in the transmutation of forms. 



Nageli and his modern successors conceive of phyletic evolution 

 as depending upon definitely directed variation, resulting from internal 

 causes and occurring at definite times, which of necessity causes 

 the existing form to be transformed into a new one. To them the 



