DIVERGENT EVOLUTION THROUGH SEGREGATION. 291 
sity of Evolution under one Set-of External Conditions, 1 spoke of this 
principle of divergence as “separation with variation; ” but in order 
to distinguish the antecedent condition, which is separation, from the 
result, which is something more than variation, | now call the certainty 
that some form of divergent transformation will arise when inter-genera- 
tion is prevented the principle of intension ; and segregation produced 
by independent transformation I call intensive segregation. 
As separate and segregate generation are so closely related, I have, 
in order to avoid a multiplication of terms, classified the two principles 
together under the general term segregation. In my discussion of the 
‘auses of segregation I shall however endeavor to determine concern- 
ing each class of causes whether they are primarily separative or seg- 
regative. 
A full discussion of the causes of segregation would require that under 
each combination of causes to which we give a distinctive name we 
should show: 
(1) How the independent generation is produced. 
(2) How the difference of character in the different sections is pro- 
duced. 
(3) How the aggregation in place bringing together the members of 
each section is produced. 
(4) How the correspondence in times and seasons necessary for inter- 
generation is secured within each section. 
(5) How the correspondence of community and of sexual and social 
instincts necessary for intergeneration is secured within each section. 
(6) How the correspondence in structure, in dimensions, and in the 
mutual potentiality of the sexual elements necessary for intergenera- 
tion is secured within each section. 
It will however be observed, that with the exception of the two 
first, these questions relate to the necessary conditions that must always 
exist in the case of every inter-generating group; and as it is evident 
that inter-generation in some degree must be the normal condition in 
every sexual, that is, in every gamo-genetic species, we may here assume 
that all the conditions necessary to inter-generation exist, except so far 
as they have been disturbed by causes producing segeneration. In 
tracing the causes of segregation it will therefore be sufficient if in 
ach class of cases we give the cause of se-generation, showing why the 
same cause does not prevent all inter-generation, and explain the differ- 
ence of character in the different sections produced by the se-genera- 
tion. In full accord with the implications of the theory of evolution, 
we proceed on the assumption that inter-generation was the original 
condition of every species, and that the inter-generation of those that 
are brought together under favorable 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 
conditions on which free crossing depends, in such a way and to such 
