The Making of Species 
upon the importance of the principle of isolation, 
rendered valuable service to biological science, 
but, in common with most men having a new 
theory, they have pushed their conclusions to 
absurd lengths. 
As Romanes has pointed out, isolation may 
be discriminate or indiscriminate. “If,” he 
writes, on p. 5 of vol. iii. of Darwin and after 
Darwin, ‘“‘a shepherd divides a flock of sheep 
without regard to their characters, he is isolating 
one section from the other indiscriminately ; but 
if he places all the white sheep in one field, and 
all the black sheep in another field, he is isolating 
one section from the other discriminately. Or, if 
geological subsidence divides a species into two 
parts, the isolation will be indiscriminate ; but if 
the separation be due to one of the sections 
developing, for example, a change of instinct 
determining migration to another area, or occu- 
pation of a different habitat on the same area, 
then the isolation will be discriminate, so far as 
the resemblance of instinct is concerned.” 
Other names for indiscriminate isolation are 
separate breeding and apogamy. Discriminate 
isolation is also called segregate breeding and 
homogamy. The human breeder resorts to 
discriminate isolation in that he separates all 
those creatures from which he seeks to breed, 
from those from which he does not wish to 
breed. Natural selection itself is, therefore, a 
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