IS'TENSTYE SEGEEGATION. 369 



the same as that wliich exists between the 13- and 17-year races. 

 Two or three of the States have but one brood each ; but in Ohio 

 ?eYen 17-year broods are reported, and iu North Carolina one 13- 

 year and six 17-year broods. I judge, however, from the reports 

 that, even in these last-mentioned States, there are but few places, 

 if any, where moi*e than three broods overlap. 



I have not seen any discussion of the causes that have pro- 

 duced these broods, but if we may believe that they have existed 

 for a thousand generations, a possible if not 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 

 wliich were too cold for their development at the return of the 

 17-year jjeriod when the offspring should have appeared; and 

 still some of tlie benumbed and delayed pupae may have survived, 

 making their appearance one, two, three or more years later, 

 when conditions were more favourable. The following observa- 

 tion referred to by Dr. E-iley, in explanation of the accelerated or 

 retarded appearance of sporadic individuals, throws some light on 

 the origin of the different broods: — "That circumstances favour- 

 able or otherwise may accelerate or retard their development was 

 accidentally proven in 1868 by Dr. E. S. Hull, of Alton, 111., as 

 by constructing 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 iu small numbers, and more rarely in large 

 numbers, the year before or the year after their proper period. 

 This is more especially the case with the 13-year broods " *. That 

 climate has been an important factor in the development of the 

 13- and 17-year races is indicated by the fact that most of the 

 districts occupied by the l7-year race lie north of lat. 38°, and 

 most of those occupied by the 13-year race lie south of that line, 

 though in Illinois there is a 13-year brood as far north as lat. 

 40°. Dr. Riley has not referred to the coincidence, but it seems 

 to me a fact of some interest in this connection, that the southern 

 limit of the great ice-cap which covered Canada and the northern 

 part of the United States during the Glacial epoch extended along 



* U.S. Department of Agriculture (Di-^ision of Entomology), Bulletin No. 8, 

 p. 8, by C, V. Eiley, 



