174 APPENDIX I DIVERGENT EVOLUTION. 



clearest illustration of this is found in the case of plants that are fer- 

 tilized 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 cer- 

 tainty, promptness, and efficiency, on whatever stigma it might fall. 

 We may also be sure that with organisms that depend upon water for 

 the distribution of their fertilizing elements, impregnational segrega- 

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

 lower taxonomic groups. 



It is important to observe that, in the cases under consideration, 

 the inferior fertility or -vigor resulting from the crossing of the incom- 

 patible forms is as truly a cause of divergence as the inferior oppor- 

 tunity for crossing which from the first existed between the members 

 occupying different localities or between flowers growing on different 

 trees of the same species. The former has been called negative and 

 the latter positive segregation, not for the sake of distinguishing 

 different grades of efficiency, but for the sake of indicating the different 

 methods of operation in the two classes of segregation. 



2. Isolation Usually Somewhat Discriminate, and therefore Segregative, 



from the First. 



Of the twenty-one natural forms of isolation enumerated in this 

 paper, there are only two that are usually indiscriminate in their 

 action. These are transportational segregation and geological segre- 

 gation. And even these sometimes become discriminate in their 

 action through the fact that those individuals that -are similarly 

 endowed are liable to be transported in the same way and to the same 

 place, or to escape together from destruction in geological disturb- 

 ances. Again, it may happen that by gradual subsidence a large 

 island will be divided into two smaller islands, and thus certain 

 species inhabiting the original island may be indiscriminately isolated . 

 But even in such a case, unless the average inheritable character of 

 each section of the species is exactly the same in all respects, the effect 

 is segregative from the first. If one, or both, of the sections is very 

 small, the probability of exact similarity in all respects entirely dis- 

 appears, unless the species is wanting in plasticity and variability. 



3. Principles Intensifying Segregation. 



Besides artificial and institutional segregation, which depend on 

 the rational purpose of man, we have now considered 2 1 forms of seg- 

 regation, resting on purely natural causes. 



At some other time I shall endeavor to present the natural laws that 

 cooperate in intensifying the effects produced by the segregative 



