234 APPENDIX II INTENSIVE SEGREGATION. 



have not been seriously changed since the close of the last glacial 

 period. Again, one generation of the seventeen-year race of Cicada 

 covers many generations of the Basilarchia, bringing thirty or forty 

 fluctuations of climate, food, etc., to the latter, while the former is, for 

 the most part, protected from serious fluctuations. 



It is of course equally impossible to prove by all inclusive observa- 

 tions either that transformation is never completely parallel in sections 

 of a species that are prevented from crossing or that independent gene- 

 ration long continued is sure to result in independent transformation, 

 and, therefore, in divergence; but it is of no small interest that we 

 find in the thirteen-year and seventeen-year races of this species the 

 strongest proof that there are sometimes divergences which our senses 

 do not perceive. If our senses were a sufficient test, it might be main- 

 tained that between these races a high degree of local and cyclical 

 isolation has existed for many generations, without any other form of 

 transformation having arisen to increase the divergence ; but if our 

 informants are correct when they tell us that these races do not cross 

 when appearing in the same district and at the same time, we need not 

 hesitate to affirm that there must be some distinguishing character- 

 istics by which those of one race are able to find each other, as well 

 as segregative instincts which lead them to choose each other's 

 society ; and, even if our informants are mistaken in supposing that 

 cross-unions do not occur, there must be some form of incompatibility 

 between the two races, resting on divergent endowments; for other- 

 wise we should find hybrid descendants with periods of more than 

 thirteen and less than seventeen years' duration. 



IV. CONCLUDING REMARKS. 



1. Outline of the Argument in Support of the Theory of Divergent Evolution 

 through Cumulative Segregation. 



(i) The invariable experience of mankind in producing domestic 

 races shows that segregation is a controlling factor. The segregation 

 that produces domestic breeds and races is found to be of two kinds : 

 first, that which is produced by men who designedly preserve the 

 different styles of variation presented by one species, while at the 

 same time they prevent them from crossing ; and, second, that which 

 commences in the indiscriminate division of the species into sections 

 that are prevented from freely crossing through their being under the 

 care of separate tribes of men, and which is changed into decided 

 segregation through the diversity of selection, or of some other trans- 

 forming principle, to which the different sections are sure to be 



