268 Evolution and Adaptation 
Pearson points out further that, even if we suppose that a 
secular change is produced in a new environment, we cannot 
explain how species may break up into two or more races 
that are relatively infertile. Suppose two groups of individ- 
uals, subjected to different environments, become isolated 
geographically. Two local races will be produced. “ Isola- 
tion may account for the origin of local races, but never for 
the origin of species unless it is accompanied by a differen- 
tial fertility.” In other words, Pearson thinks that, unless 
the reproductive organs are correlated with other organs, in 
such a way that as these organs change the interracial fer- 
tility of the germ-cells is altered, so that in the two changed 
groups the individuals are no longer interfertile, new species 
cannot be accounted for, since their mutual infertility is one 
of their most characteristic features. ‘Without a barrier to 
intercrossing during differentiation the origin of species 
seems inexplicable.” 
We need not discuss the various suggestions that have 
been made to explain this difficulty, none of which, as Pear- 
son points out, have been satisfactory. He himself believes 
that a process of segregation of like individuals must occur, 
during the incipient stages at least, in the formation of species. 
Afterwards a correlation may exist between the new organs 
and the germ-cells, of such a sort that a relative or an abso- 
lute sterility between the incipient species is attained. After 
this condition has been reached the two new species may 
freely intermix without a return to the primitive type, since 
they are no longer fertile zzter se. It seems to me, also, that 
this would be an essential requisite if we assume that species 
are slowly formed out of races from individual differences, as 
Pearson supposes to be the case. There are, however, other 
possibilities that Pearson does not take into account, namely, 
that from the very beginning the change may be so great 
that the new form is not fertile with the original one; and 
there is also another possibility as well, that, although the 
