THE ORIGIN OF THE BROODS. 19 
case, the Cicada would have appeared everywhere over its range in the 
same year and probably at about the same time. In the long course of 
ages, with the consequent important changes — geographic, climatic, 
and topographic — this original brood became gradually broken up into 
many broods, with constantly increasing divergence in the dates of* 
appearance, so that at the present time nearly every year has its brood. 
or broods, each of which is limited, as a rule, to well-defined district-, 
and each reappearing at the proper intervals with absolute regularity. 
Of the upward of twenty broods which have been differentiated, most 
of them have been carefully studied, chronological records collected, 
and the limits of distribution fairly well determined. For convenience 
of reference, these broods have been designated by Roman numerals 
as Brood X, Brood XVII, etc. 
The origin of distinct broods in an insect possessing as long a 
developing period as the one under discussion is not difficult of expla- 
nation. It is a well-known phenomenon in connection with insect life 
that, whatever may be the period of development of a species, certain 
individuals will often, for some reason or other, such as insufficient 
or unsuitable food, unfavorable temperature, or other conditions, be 
delayed or retarded, while others, for reasons the converse of the last. 
namely, conditions exceptionally favorable, will develop more rapidly 
or will be accelerated and appear earlier. Therefore, under the former 
conditions we have a longer and under the latter conditions a shorter 
life period. 
This is true to a slight degree at the present time of the periodical 
Cicada, and especially with the larger broods has it been noticed that 
scattering individuals appear the year before and others the year after 
the great brood year. It is not difficult to imagine, therefore, that 
under exceptional conditions some of the earlier appearing individuals 
or the later ones may occur in sufficient numbers to establish a well- 
marked peculiarity in this direction and form a new brood appearing a 
year earlier or a year later than the original one. If in the long course 
of years some accident should happen to the parent brood in that 
portion of its range the derivative brood might be left to hold the 
territory alone or to become the predominant swarm. 
This explanation is supported also by the fact that it often happens 
that the broods of two successive years occupy contiguous territory. 
as, for example, the 13-year Brood VI, which last appeared in 1897, is 
distributed between Vicksburg and New Orleans, or just south of the 
13-year brood of the present year. It is reasonable to infer, therefore, 
that Brood VI is simply a strong, well established colony of accelerated 
individuals from the southern end of Brood VII, with a 13-year period 
terminating one year earlier than that of the parent brood. The cow- 
ditions which led to the emergence of the insect below Vicksburg in 
twelve years some time in the remote past being temporary, this portion 
of the old brood resumed the normal 1.5 year period. 
