230 APPENDIX ii INTENSIVE; SEGREGATION. 



a special study of the subject that they do not cross when these 

 opportunities occur; for 14, 15, and 16 year races are not found. 

 These two races are, therefore, prevented from crossing by partial 

 local isolation ; by cyclical isolation rendering it impossible that a 

 brood of each occupying the same locality should have opportunity 

 for crossing more than once in seventeen generations of the shorter- 

 lived race, or once in thirteen generations of the longer-lived race ; and 

 by sexual isolation that shows itself in diversity of instincts preventing 

 them from pairing when other conditions favor. 



Whether devices have been tried to induce cross-unions, and whether 

 such unions are unfruitful, I have never heard ; but the simple fact 

 that fifteen-year forms do not appear in localities where the two races 

 are found indicates that in nature they do not cross. Several such 

 localities have been reported, but in none of them has an intermediate 

 form been found. It seems, therefore, that we may safely draw the 

 conclusion that we have here a case of complete sexual segregation be- 

 tween forms which to the human eye are undistinguishable, and which 

 call their mates with stridulations which to the human ear are the same. 

 Now, I claim that in such races as these we have the beginning of diver- 

 gent species, a beginning that lies in the segregative influences of con- 

 stitutional and instinctive qualities persistently inherited by the two 

 races, though the naturalist who examines specimens of the two races 

 can not distinguish them. All that is necessary to convert these two 

 races into good species is the transformation of one or both of them 

 while they are thus prevented from crossing; for we may be assured 

 that the results of transformation under such circumstances will never 

 be completely parallel. 



Each of these races is again subdivided ; for accompanying each is 

 a diminutive form, differing somewhat in color, not so early by eight 

 or ten days in its first appearance, producing a quite distinct stridula- 

 tion, 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 cassinii. Dr. Riley, however, hesitates to 

 receive it as a separate species, because the differences presented by 

 the 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 (septendecim) are quite constant in their appearances, 

 those of the smaller kind (cassinii) are variable, and in some few specimens are 

 indistinguishable from those of the large kind. This circumstance, coupled with 

 the fact that the small kind regularly occurs with both the seventeen and thir- 

 teen year broods, would indicate it to be a dimorphic form of the larger, and only 

 entitled to varietal rank.* 



* Bulletin No. 8, Division of Entomology, U. S. Department of Agriculture, p. 7. 



