SEXUAL INCOMPATIBILITY. I 73 



segregation of a rigorous form (as, for example, complete geograph- 

 ical segregation), considerable divergence may result without any 

 sexual incompatibility. Darwin has shown, by careful experiments, 

 that integrate vigor and fecundity is the relation in which the varieties 

 of one species often stand to each other. This fact does not, how- 

 ever, prove that the more strongly divergent forms, called species, 

 which are prevented from coalescing by segregate vigor and fecundity, 

 did not acquire some degree of this latter character before anv perma- 

 nent divergence of form was acquired. Their having acquired this 

 segregating characteristic may be the very reason why their forms are 

 now so decidedly different, for without it they would have been swal- 

 lowed up by the incoming waves of intergeneration. Again, we must 

 remember that forms only moderately divergent are habituallv 

 classed as different species if they are separated by segregate vigor 

 and fecundity (that is,' by some degree of mutual sterility), unless 

 observation shows that they are of common descent. These two 

 considerations sufficiently explain why the varieties of one species 

 are so seldom reported as mutually infertile. Notwithstanding this, 

 the experiments of Gartner and of Darwin seem to show that seg- 

 regate fecundity and vigor may arise between varieties that spring 

 from one stock. In view of these cases we must believe that in 

 the formation of some, if not many species, the decisive event 

 with which permanent divergence of allied forms commences is the 

 intervention of segregate fecundity or vigor between these forms. 

 Positive segregation, in the form of local, germinal, or floral segrega- 

 tion, producing only transitory divergences, always exists between the 

 portions of a species that has many members; but as it does not 

 directly produce the negative segregation which is, in such cases, the 

 necessary antecedent of permanent divergence, we can not, in accord- 

 ance with the usage of language, call it the cause of the permanent 

 divergence. Moreover, though it may be in accordance with ordinary 

 language to call the negative segregation, which is the immediate 

 antecedent of the permanent divergence the cause of the same, it will 

 be more correct to call the coincidence of the negative and positive 

 segregations the cause, and still more accurate to say that the whole 

 range of vital activities (when subjected to the limitations of anv 

 sexual incompatibility that corresponds in the groups it separates to 

 some previous but ineffectual local, germinal, or floral segregation) 

 will produce permanent divergence. 



Tn manv cases not only is the entrance of impregnational segrega- 

 tion the cause of the commencement of permanent divergence, but its 

 continuance is the cause of the continuance of the divergence. The 



