THEOUGH CUMULATIVE SEGREGATION. 



217 



the causes of Segregation it will therefore be sufficient if in 

 each class of cases we give the cause of Segeneration, showing 

 why the same cause does not prevent all intergeneration, and 

 explain the difference of character in the different sections pro- 

 duced by the Segeneration. In full accord with the implications 

 of the theory of evolution, we proceed on the assumption that 

 intergeneration was the original condition of every species, and 

 that the intergeneration of those that are brought together under 

 favourable circumstances may be taken for granted, unless there 

 is some special cause that prevents. All that is necessary to 

 produce Separation is the failure of any one of the many con- 

 ditions on which free-crossing depends, in such a way, and to 

 such a degree, that the species falls into two or more sections 

 between which crossing is interrupted, without its being inter- 

 rupted within the bounds of each section. And all that is 

 necessary to produce Segregation is that to Separation should be 

 added some cause that secures difference of character in the 

 different sections. And as Separation long continued inevitably 

 ends in Segregation through the development of difference of 

 character in the different sections, we need not in our classifi- 

 cation set them wholly apart, though for the sake of clearly 

 recognizing the difference it will be well to note in each class of 

 causes whether the primary effect is Separation or Segregation. 



Cumulative Segregation and the Classification of 

 ITS Different Forms. 



The fundamental law to which I would call attention may be 

 expressed in the following formula : — Cumulative Segregation 

 produces accumulated divergence; and accumulated divergence 

 produces permanent Segregation ; and the Segregate subdivision 

 of those permanently Segregated produces the divisions and sub- 

 divisions of organic phyla. If, then, we can discover the causes 

 of Segregation, we shall understand the causes of a wide range 

 of phenomena ; for this is the fundamental principle in the for- 

 mation of varieties, species, genera, families, orders, and all greater 

 divergences that have been produced in the descendants of 

 common ancestry. 



In treating of the causes of Segregation, I have found it con- 

 venient to make two distinct classifications. In the one the 



