THROUGH CUMTJLATITE SEGEEOATIOIS'. 



271 



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, already referred to at length, seem to 

 show that Segregate Fecundity and Vigour 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 

 A^igour between these forms. Positive Segregation, in the form 

 of Local, Grerminal, or Floral Segregation producing only tran- 

 sitory 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 ante- 

 cedent of permanent divergence, we cannot, in accordance with 

 the usage of language, call it the cause of the permanent diver- 

 gence. 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 limita- 

 tions of any sexual incompatibility that corresponds in the groups 

 it separates to some previous but ineffectual Local, Grerminal, 

 or Floral Segregation), will produce permanent divergence. 



In many cases not only is the entrance of Impregnational 

 Segregation the cause of the commencement of permanent diver- 

 gence, but its continuance is the cause of the continuance of the 

 divergence. The clearest illustration of this is found in the case 

 of plants that are fertilized by pollen that is distributed by the 

 wind. All the higher, as well as the lower, groups of such plants 

 would rapidly coalesce if each grain of pollen was capable of 

 producing fertilization, with equal certaint}'-, promptness, and 

 efficiency, on whatever stigma it might fall. We may also be 

 sure that, with organisms that depend upon water for the dis- 

 tribution of their fertilizing elements, Impregnational Segrega- 

 tion is an essential factor in the development of higher as well 

 as of lower taxouomic groups. 



It is important to observe that, in the cases under considera- 

 tion, the inferior fertility or vigour resulting from the crossing of 



