INTENSIVE SEaEEGATION". 



367 



and instinctive qualities persistently iulierited by the two races, 

 though the naturalist who examines specimens of the two races 

 cannot distinguish them. All that is necessary to convert these 

 two races into good species is the transformation o£ one or both 

 of them while they are thus prevented from crossing ; for we may 

 be assured that the results of transformation under such circum- 

 stances will never be completely parallel. 



Each of these races is again subdivided ; for accompanying 

 each is a diminutive form, differing somevfhat in colour, not so 

 early by eight or ten days in its first appearance, producing a 

 quite distinct stridulation, and showing no disposition to associate 

 with the larger form. This small form was described in 1851 by 

 Dr. Fisher as a new species under the name Cicada Gassinii. Dr. 

 Riley, however, hesitates to receive it as a separate species 

 because the differences presented by the male genitalia are not 

 constant. He says " there are sufficient differences to separate 

 the two forms as distinct ; but while the hooks of the large kind 

 {septemdecim) are quite constant in their appearances, those of 

 the smaller kind {Gassinii) are variable, and in some few specimens 

 are indistinguishable from those of the large kind. This cir- 

 cumstance, coupled with the fact that the small kind regularly 

 occurs with both the 17- and 13-year broods, would indicate it to 

 be a dimorphic form of the larger, and only entitled to varietal 

 rank " *. 



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

 for it is an example of complete segregation between tlie forms 

 of one species through diversity in their instincts. "Whether 

 these divergent instincts 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 pro- 

 duced 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 con- 

 sidered something more than a dimorphic form of the large one. 

 It would, in that case, be a dimorphic form that is assuming the 



* United-States Department of Agriculture (Division of Entomology), Bul- 

 letin No. 8, p. 7. 



