368 EEV. J. T. GULICK ON 



character of a species. If the two forms were without segregative 

 sexual and social instincts, then, with cross-fertility, the small 

 form would be rapidly absorbed by the large form, wKich greatly 

 preponderates in numbers ; and with cross- sterility the small form 

 would rapidly become extinct ; for, through the comparative 

 scarcity of their numbers, the representatives of the small form 

 would have but little chance of mating with each other. 



On the other hand, if the Sexual and Social Segregation is 

 complete, it matters but little whether the forms are mutually 

 sterile, for the separate races or species will be protected by the 

 Positive Segregation produced hy the divergent instincts, even if 

 the Negative Segregation, depending on structural iucompatibility 

 and lack of physiological adaptation, is entirely wanting. It is 

 only when associated witli Positive Segregation that is partial in 

 its results, that Negative forms of Segregation become important 

 factors in the preservation of diverging forms. 



In animals that pair, Segregation through sexual and social 

 instincts plays a similar role in giving pre-emptive power to the 

 males of a given species over the females of the same species, that 

 is played by Potential and Prepotential Segregation in organisms 

 whose fertilizing elements are distributed by wind or water. In 

 the one case Instinctive, and in the other Potential Segregation, 

 arising between varieties of the same species, marks these varieties 

 as being the initial forms of divergent species. 



This species presents another form of Segregation which is of 

 much interest, though it has not yet resulted in forms that can 

 be ranked as different races. I refer to the complete Cyclical 

 Segregation that exists between the dilFerent broods of a given 

 race appearing in different years. Of the 13-year race there are 

 seven broods, and of the 17-year race fourteen. As an example 

 of different broods occurring in the same region I would mention 

 the two broods in the district of Columbia, one appearing in 1885 

 and at intervals of seventeen years thereafter, and another ap- 

 pearing in 1894 and at intervals of seventeen years thereafter. 

 We have no means of testing the sexual or social instincts of 

 these different broods, for they never appear in the same year. 

 No one can say whether if they could be brought together they 

 would be found as indisposed to breed with each other as are the 

 13-year and 17-year races. But, be that as it may, the two forms 

 are as completely segregated as they can be, and the opportunity 

 for independent, and therefore divergent, transformation is much 



