300 DIVERGENT EVOLUTION THROUGH SEGREGATION. 
be separated from all companions by its industrial habit, it is usually 
found that those that at one time and one place adopt the habit are 
usually sufficient to keep up the new strain, if they succeed in securing 
the needed sustenance. 
(b) Chronal segregation. 
is segregation arising from the relations in which the organism stands 
to times and seasons. 
I distinguish two forms—cyclical and seasonal segregation. 
4, Oyclical segregation is segregation arising from the fact that the 
life cycles of the different sections of the species do not mature in the 
Same years. 
A fine illustration of this form of segregation is found in the case of 
Cicada septemdecim, whose metropolis is in Virginia, Maryland, and 
Delaware, though many outlying broods are found in other regions 
east of the Mississippi River. The typical form has a life-cycle of sey- 
enteen years, but there is a special race (Cicada tredecim, Riley) that is 
separated from the typical form, both locally and chronally. As the 
life-cycle of this race is thirteen instead of seventeen years, even if oc- 
cupying the same districts and breeding at exactly the same season, 
inter-breeding could occur between the two forms only once in two hun- 
dred and twenty-one years, or once in thirteen generations of the longer 
lived race, and once in seventeen generations of the shorter lived race. 
During the year 1885 the two races appeared simultaneousiy. The op- 
portunity for testing whether they would freely interbreed if brought 
together has therefore passed not to return till the year 2106; but the 
distribution of the two races in different districts seems to indicate 
that local segregation has had an important influence in the develop- 
ment of the race. It is manifest, however, that if during a period of 
local separation, or if during the period of two hundred and twenty- 
one years of cyclical separation after the thirteen-year race was first 
formed, this race should become modified in the season of its appear- 
ing, there would after that be no mingling of race, though brought to- 
gether in the same districts. This would be seasonal segregation, 
which we shall consider in the nextsection; but what is of special inter- 
est here, as an example of complete cyclical] segregation, is the fact that 
at Fall River, Mass., there is a brood of the septemdecim form, due a 
year later than the universal time of appearing.* 
In any species where the breeding of each successive generation is 
separated by an exact measure of time which is very rigidly regulated 
by the constitution of the species, cyclical segregation will follow, if 
through some extraordinary combination of circumstances, members 
*See statement by Prof. C. V. Riley, in Science, vol. v1, p.4. For particulars con- 
cerning the distribution and habits of this species, see a paper by Prof. Riley, read 
before the Biological Society of Washington, May 30, 1885, extracts from which are 
given in Science, vol. Vv, p. 518. 
