THROUGH CUMULATIYE SEGREGATION. 



227 



species, and has availed itself of the protection from the weather 

 offered by the eaves of civilized houses ; and that with this 

 change in nest-building has come a change in some of its other 

 habits. Now there is reason to believe that if the number of 

 houses had been limited to a hundredth part of those now exist- 

 ing, and if that limited number had been very slowly supplied, 

 this gradual change in some of the elements of the environment 

 would have resulted in divergent forms of adaptation to the en- 

 vironment in two sections of the same species. One section 

 would have retained the old habit of building in the cliffs, with 

 all the old adaj)tations to the circumstances that depend on that 

 habit ; while another section of the species would have availed 

 itself of the new opportunities for shelter under the eaves of 

 houses, and would have changed their inherited adaptations to 

 meet the new habits of nest-building and of feeding. It is also 

 evident that the prevention of free interbreeding between the 

 different sections caused by the diversity of habits would have been 

 an essential factor in the divergence of character in the sections. 



It simply remains to consider whether the industrial habit 

 that separates an individual from the mass of the species will 

 necessarily leave it alone, without any chance of finding a consort 

 that may join in producing a new intergenerant. The answer is 

 that there is no such necessity. Though it may sometimes 

 happen that an individual may be separated from all companions 

 by its industrial habit, it is usually found that those that at one 

 time and one place adopt the habit are usually sufficient to keep 

 up the new strain, if they succeed in securing the needed sus- 

 tenance. 



(b) Chronal Segregation 



is Segregation arising from the relations in which the organism 

 stands to times and seasons. 



I distinguish two forms — Cyclical and Seasonal Segregation. 



4. Cyclical Segregation is Segregation arising from the fact 

 that the life cycles of the different sections of the species do not 

 mature in the same years. 



A fine illustration of this form of Segregation is found in the 

 case of Cicada sepiemdecim, wliose metropolis is in Virginia, 

 Maryland, and Delaware, though many out-lying broods are 

 found in other regions east of the Mississippi Eiver. The typical 

 form has a life-cycle of seventeen years, but there is a special 



