THEORY OP A PHYLETIC FORCE 131 



tion. Selection, again, intervenes in consanguinity, rooting out 

 those families whose consanguinity has been the cause of their 

 degeneracy, preserving those whose consanguinity has been bene- 

 ficial. In the same way, we may say that there is selection of 

 first crosses and of hybrids. And now we come to see that in 

 the origin and evolution of species in general selection is the pre- 

 ponderating — nay, the sole — factor ; for species may be defined 

 in every case as complexes of adaptation. 



A. — Species as determined by Selection. 



The idea of a " phyletic force," which was suggested by Nageli, 

 is based on the supposition that every species is, as it were, a 

 living crystal, determined in its evolution by internal forces, each 

 step in a given direction necessarily producing the next step. 

 For Nageli the role of natural selection in the evolution of 

 species was practically reduced to zero ; for Nageli would not 

 admit that the modifications of a species are due to adaptation, 

 or that they were anything but the result of the action of intra- 

 germinal forces. Accordingly, once this view of evolution is 

 accepted, there is nothing to prevent our drawing the conclusion 

 which Nageli drew — ^which, indeed, is the only logical conclusion 

 — namely, that the environment is without influence on the 

 evolution of species, and that their constitution at any given 

 moment would remain identical under entirely different con- 

 ditions. It is a " phyletic force " which guides the evolution of 

 the organic world, and which produces those transformations 

 which result in the differentiation of weU-defined species. It 

 should be noted that Nageli thought of this phyletic force in a 

 purely scientific and mechanical way, not by any means in a 

 mystical one. Nevertheless, the theory of Nageli requires to be 

 completed and explained by that of Darwin. 



Were the theory of Nageli sufficient, it would be easy to under- 

 stand how it is that we find everywhere the aggregates which we 



9—2 



