BROODS AND RACES OF THE PERIODICAL CICADA. 23! 



I consider this case as of equal interest with the previous one ; for it 

 is an example of complete isolation between the forms of one species 

 through diversity in their instincts. Whether these divergent in- 

 stincts are sexual or social may be a matter of question ; but in either 

 case they are effectual in preventing crossing. 



If future investigation shows that the small form is often produced 

 directly from the eggs of the large form, it will have but little claim to 

 be regarded as a separate race ; but even then, if the small form breeds 

 only with its own kind, as has been reported by several observers, and 

 if the offspring persistently reproduce the characters of the parents, 

 it will have to be considered something more than a dimorphic form 

 of the large one. It would, in this case, be a dimorphic form that is 

 assuming the 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, which 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 isolation is complete, 

 it matters but little whether the forms are mutually sterile, for the sep- 

 arate races or species will be protected by the positive segregation 

 produced by the divergent instincts, even if the negative segregation 

 depending on structural incompatibility and lack of physiological 

 adaptation is entirely wanting. It is only when associated with pos- 

 itive 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, isolation through sexual and social instincts 

 plays a similar role in giving preemptive power to the males of a given 

 species over the females of the same species that is played by poten- 

 tial and prepotential isolation in organisms whose fertilizing elements 

 are distributed by wind and 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 isolation which is of much in- 

 terest, 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 different broods of a given race appearing in different years. 

 Of the thirteen-year race there are seven broods and of the seven- 

 teen-year race fourteen. As an example of different broods occurring 



