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 atintervalsof 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. Noonecan 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 
greatice 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 pupz 
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, Ill., as by con- 
structing underground flues for the purpose of forcing vegetables, he also caused 
the Cicadas to issue as early as the 20th 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. 
