20 THE PERIODICAL CICADA. 
Another marked instance of the same kind is shown in the relations 
between Brood I and Brood XXII, the former being merely an appen- 
dix or a continuation in a northeasterly direction of the territory occu- 
pied by the eastern branch of Brood XXII, which always precedes 
Brood I by one year. The interrelations of these and all the other 
broods are indicated in the discussion of the distribution of the Cicada. 
Local or temporary conditions which have caused a moderate change 
in the time of emergence of the Cicada are on record, one notable 
instance resulting from an artificial heating of the soil by hot pipes 
(see p. G2). A similar instance is suggested by Mr. Schwarz. 1 Com- 
menting on the slightly earlier emergence of individuals of Brood VIII 
near Harpers Ferry, W. Va., in 1889, in a small clearing surrounded by 
woods, Mr. Schwarz urges that a clearing made in the midst of a dense 
forest forms a natural hothouse, the soil receiving in such places much 
more warmth than in the shady woods. That the Cicadas should 
appear a little earlier in such situations is not remarkable, and he sug- 
gests also that uuder favorable circumstances the Cicada might develop 
on such cleared places one or several years in advance of the normal 
time, and that these precursors, if numerous enough, would be able to 
form a new brood. 
It is possible to conceive also of conditions which would result in the 
acceleration or retardation in the development of an entire brood or 
broods of the Cicada, such as variation in climatic conditions, geolog- 
ical changes, or changed conditions of the topography of the country, 
including the character of the vegetation. 
In this or other ways, at any rate, the Cicada has become broken up 
into a large number of distinct broods, often covering different territory, 
but not necessarily so doing, each, however, maintaining absolutely its 
regular time of appearance. 
The slight but constant tendency to variation which has brought into 
existence the Droods now so well marked, continued indefinitely, would 
so break up and scatter the present broods as to ultimately obscure 
them altogether, and the overlapping of districts and the variation in 
time of appearance would lead to a rather general occurrence every 
year of the periodical Cicada throughout its range, the long period for 
development, however, still persisting. Anticipating such an outcome 
from the intermixture and overlapping merely of different broods, Dr. 
Smith (Smith MS.) rather mournfully says: "In those times, if these 
sayings of mine should be thought of, they will be ridiculed as a super- 
stitious legend of the olden times." 
THE CLASSIFICATION OF THE BROODS. 
The earlier writers on this insect, as Prof. Nat. Potter, Dr. Harris, 
and Dr. Smith, classified the broods solely according to the years 
of their appearance. In Smith's unpublished register every year is 
1 Proc. Eut. Soc. Wash., I, p. 230. 
