INTENSIYE SEGREGATION. 371 



ditions have not been seriously changed since the close of the 

 last glacial jDeriod. Again, one generation of the 17-year race of 

 Cicada covers a period equal to that of thirty or forty generations 

 of the Basilarclna, bringing thirty or forty fluctuations of 

 climate, food, &c. 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 obser- 

 vations, either that transformation is never completely parallel iu 

 sections of a species that are prevented from crossing, or that inde- 

 pendent generation 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 13-year and 17-year races of this 

 species the strongest proof that there are sometimes divergences 

 which our senses do not perceive. If our senses were a sufiicient 

 test, it might be maintained that between these races a high 

 degree of local and Cyclical Segregation has existed for many 

 generations, without any other form of transformation having 

 arisen to increase the divergence ; but if oiir 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 cha- 

 racteristics by which those of one race are able to find each other, 

 as well as segregative instincts whicb lead them to choose each 

 other's society; and, even if our informants are mistaken in sup- 

 posing that cross-unions do not occur, tbere must be some form 

 of incompatibility between the two races, resting on divergent en- 

 dowments ; for otherwise we should find hybrid descendants with 

 periods of more than 13 and less than 17 years' duration. 



Concluding Eemarks. 



Outline of tlie Argument in support of the Theory of Divergent 

 JEvolution through Cumulative Segregation. 



(1) The invariable experience of mankind in producing do- 

 mestic 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 iu the indiscri- 



