2 



arranged in a series in accordance with their sequence in time, an 

 important natural relationshij^ in point of origin and distribution is 

 plainly indicated. 



Taking first the broods of the 17-year race, it will be seen from 

 the subjoined table that if the enumeration begin with Brood XI, 

 the 17-year broods follow each other in regular succession for eleven 

 consecutive years ; then after a break of one year follows Broods V 

 and VIII, and after another break of one year. Brood IX ; another 

 break of one year precedes the next recurrence of Brood XI, with 

 which the series starts. 



Chrouoloijical order of the broods of the Cicada from 1S93 to 1910. 



Taking up the 13-year broods in the same way, it will be seen that 

 if the enumeration start with Brood XVI, a 13-year brood follows in 

 regular succession for six years. With the exception of the Y&vy 

 doubtful Brood X, which is separated from the last 13-year brood 

 by three years, there follow seven successive years in which no 

 13-year broods occur. 



Under the supposition that the different broods of the 17-year and 

 13-year races sprang in the remote past from an original brood of 

 each, it would naturally follow that the broods most closely related 

 in time would also present a closer relatioftship in their range, and 

 this, in fact, proves to be generally true. 



To show this relationship and to indicate the natural order of their 

 occurrence, I have to suggest a new enumeration of the broods in 

 which the two races are separated — the 17-year broods coming first, 

 followed, for convenience merely, by the 13-year broods. Thus 

 Brood XI of the 17-year race becomes Brood I, and the others are 

 numbered in the regular order of their occurrence, except that I 

 have assigned a brood number to each of the seventeen years. This 

 leaves Broods XII, XV, and XVII, as newly numbered, without 

 any definite colonies, so far accepted, as representatives of estab- 

 lished broods. As will be shown later, however, there are records 

 which indicate the existence of small or scattering broods filling the 

 three gaps mentioned in the 17-year series. 



In the renumbering the broods of the 13 -year race I have contin- 

 ued for convenience from the end of the series of the 17-year race, 

 the first 13-year brood becoming Brood XVIII, and I have assigned 



