26 
THE PERIODICAL CICADA. 
may be explained by the emergence of this peninsula from the ocean 
in recent geologic times and also, perhaps, to the unfavorable charac- 
ter of the soil and climate as a whole. Its western limits are central 
Colorado and western Texas. Beyond the Bocky Mountains no broods 
are known, with the exception of the doubtful brood recorded as occur- 
ring along the northern slope of the Big Horn Mountains of western 
Wyoming and Montana, on the Pacific watershed. 
The territory covered by the periodical Cicada is graphically illus- 
trated, in general view, by the two maps showing the range of the 13-year 
and 17-year races, respectively. (Figs. 2 and 3). A brief examination 
of these maps develops the very interesting and suggestive fact that if 
superimposed the areas occupied by the two races would rather accu- 
Fig. 3.— Map showing distribution of the broods of the 17-year race. 
rately fit together along their adjoining sides This is to have been 
expected, but it is nevertheless remarkable that the notable extension 
northward in Illinois and Missouri of the 13-year race should fill almost 
exactly a region little if at all occupied by broods of the 17-year race. 
This circumstance has a special significance when it is remembered that 
the northward extension of the 13-year race is based on Broods XVIII 
and VII, and that the records of the former were collected largely by 
Walsh and Eiley in 1868, when this brood was in conjunction with the 
great 17 -year Brood XXII, and of the latter by Eiley in 1885 when Brood 
VII was also in conjunction with this same Brood XXII, the limits of 
which, curiously enough, stop rather suddenly at or near the eastern 
State line of Illinois. The possibility is immediately suggested that 
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