232 APPENDIX ii INTENSIVE; SEGREGATION. 



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 one appearing in 1894 and at intervals of seventeen 

 years thereafter. We have no means of testing the sexual or the 

 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 

 thirteen-year and seventeen-year races. But, be that as it may, the 

 two forms are as completely isolated as they can be, and the oppor- 

 tunity for independent, and, therefore, divergent, transformation, is 

 much the same as that which exists between the thirteen-year and 

 seventeen-year races. Two or three of the States have but one brood 

 each ; but in Ohio there are at least six seventeen-year broods, and 

 in North Carolina one thirteen-year and six seventeen-year broods. 

 I judge, however, from the reports that even in these last-mentioned 

 States, there are but few places, if any, where more than three broods 

 overlap. 



I have not seen any discussion of the causes that have produced these 

 broods, but if we may believe that they have existed for a thousand 

 generations, a possible if not a probable cause is found in the unsettled 

 conditions of climate that must have attended the breaking-up of the 

 great ice period. During years of diminished cold, colonies may have 

 taken possession of regions which were too cold for their development 

 at the return of the seventeen-year period when the offspring should 

 have appeared; and still some of the benumbed and delayed pupae 

 may have survived, making their appearance one, two, three, or more 

 years later, when conditions were more favorable. The following ob- 

 servation referred to by Dr. Riley, in explanation of the accelerated 

 or retarded appearance of sporadic individuals, throws some light 

 on the origin of the different broods: 



That circumstances favorable or otherwise may accelerate or retard their devel- 

 opment was accidentally proven in 1868 by Dr. E. S. Hull, of Alton, 111., as by con- 

 structing underground flues for the purpose of forcing vegetables, he also caused 

 the Cicadas to issue as early as the 2oth of March, and at consecutive periods 

 afterwards till May, though, strange to say, these premature individuals did not 

 sing. They frequently appear in small numbers, and more rarely in large num- 

 bers, the year before or the year after their proper period. This is more especially 

 the case with the thirteen-year broods.* 



That climate has been an important factor in the development of the 

 thirteen and seventeen year races is indicated by the fact that most 

 of the districts occupied by the seventeen-year race lie north of latitude 



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



