THROUGH 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 Sepai’ation 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 Dorms. 
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 
