DIVERGENT EVOLUTION THROUGH SEGREGATION. 23 
and the Law of the Migration of Organisms,” an English translation 
of which was published by Edward Stanford (London, 1873), the same 
author maintains that ‘* the constant tendency of individuals to wander 
from the station of their species is absolutely necessary for the forma- 
tion of races and species” (p. 4). ‘* The migration of organisms and 
their colonization are, according to my conviction, a necessary con- 
dition of natural selection” (p. 5). On pages 66 and 67 he expands 
the same statement, and objects to Darwin’s view “that on many large 
tracts all individuals of the same species have become gradually 
changed.” Again, he contends that “transformation is everywhere 
and always dependent on isolation in order to have lasting effect. 
Without separation from the home of the species, this wonderful 
capacity would be completely neutralized” (p. 74). “Natural selection 
is not in itself an unconditional necessity, but is dependent on migra- 
tion and geographical isolation during a long period, together with 
altered conditions of life” (p. 57). “‘ Where there is no migration, that 
is, where no isolated colony is founded, natural selection can not take 
place” (p. 59). 
A comparison of his paper with my two papers published in 1872, 
already referred to, will show several fundamental differences in the 
two theories. He maintains that— 
(1) The separation of a few individuals from the rest of the species is 
absolutely necessary for the operation of natural selection, and there- 
fore for any transformation of the species, no matter how great the 
change of conditions may be in the original home of the species. 
(2) Migration and geographical barriers are the only effectual 
‘“auses, Independent of human action, by which a few individuals can 
be separated from the rest of the species, and are therefore necessary 
to the transformation of species. 
(3) Exposure to a new form of natural selection is a necessary con- 
dition for any transformation of a species. 
(4) Difference of external conditions is necessary to difference of 
natural selection, and therefore necessary to any transformation of 
species. 
(5) Geographical isolation and altered conditions of life are necessary 
conditions for natural selection, as that is for the modification of spe- 
cies. 
(6) The separation of which he speaks is the entering of a few indi- 
viduals into anew territory, where the conditions are different from 
those in the old habitat, and where the body of the species fail of reach- 
ing them. 
My chief positions were the following, in strong contrast with the fore- 
going— 
(1) Separate generation is a necessary condition for divergent evolu- 
tion, but not for the transformation of all the survivors of a species in 
one way. 
Hy. Mais,.004,. pt.. 118 
