8 INTRODUCTION. 



there is a free blending of the different types of inheritance. Now, it 

 is recognized by all that the influence of the different forms of selec- 

 tion rests on heredity ; if, therefore, we so expound natural selection 

 as to deny the fundamental laws of heredity expressed in the depend- 

 ence of divergence on segregate breeding, we undermine the founda- 

 tions on which the importance of selection rests. Clear recognition 

 of the laws of life makes it plain that a consistent theory of divergent 



* J o 



evolution can not be constructed without considering the principles 

 by which segregate breeding is modified and intensified. Those who 

 trace these principles, and who show that diversity of natural selec- 

 tion and the other forms of selection are effective in producing diver- 

 gent types only when cooperating with isolation in producing segre- 

 gate breeding, are simply recognizing the unfailing influence of hered- 

 ity. Segregation is the fundamental process to which all the princi- 

 ples helping to produce divergent evolution must contribute. 



I consider it important to recognize the very different spheres 

 filled by the principles of selection and isolation, and to so define the 

 terms that they do not overlap. But I also maintain that there is a 

 correlation in the two processes, in that both cooperate in controlling 

 heredity and variation, and so in producing ever-increasing segregation 

 of organic types. The fundamental unity of the processes producing 

 evolution, whether these processes are originated and controlled by 

 innate powers or by acquired characters, and whether these powers 

 maintain relations with other members of the species or with condi- 

 tions external to the species, is found in the fact that they all co- 

 operate in producing segregation. 



