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 inter 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 



