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 injerior 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 Furst. 
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 21 forms of seg- 
regation, resting on purely natural causes. 
At some other time I shall endeavor to present the natural laws that 
coéperate in intensifying the effects produced by the segregative 
