124 ANALYSIS OF THE FOUR*PRINCIPLES (CONTINUED). 
local separation, or if during the period of 221 years of cyclical separa- 
tion after the thirteen-year race was first formed, this race should 
become modified in the season of its appearing, there would after that 
be no mingling of race, though brought together in the same districts. 
This would be seasonal isolation, which we consider in a following 
paragraph; but what is of special interest here as examples of com- 
plete cyclical isolation is the fact that in each of several limited dis- 
tricts there are found two broods of the same race whose appearance 
above ground is always separated by the same number of years.* 
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 isolation will follow, if, 
through some extraordinary combination of circumstances, members 
sufficient to propagate the species are either hastened or delayed in 
their development, and thus thrown out of synchronal compatibility 
with the rest of the species. If, after being retarded or hastened in 
development so that part of a cycle is lost or gained, the old constitu- 
tional time measure reasserts itself, the isolation is complete. 
In such cases, so far as the time of maturing is concerned, the differ- 
ence is segregative, while in every other respect it will be simply 
separative, except as separation passes into segregation. If the 
periodical cicada wasas variable in form and color as is the Achatinella 
(as well as other genera of Hawaiian snails), we should probably find 
each brood characterized by easily recognized divergences. 
Seasonal isolation is produced whenever the season for reproduction 
in any section of the species is such that it can not interbreed with 
other sections of the species. It needs no argument to show that if, 
ina plant species that regularly flowers in the spring, there arises a 
variety that regularly flowers in the autumn, it will be prevented from 
interbreeding with the typical form. The question of chief interest 
is, under what circumstances are varieties of this kind likely to arise? 
Is a casual sport of this kind likely to transmit to subsequent genera- 
tions a permanently changed constitution? If not, how is the new 
constitution acquired? One obvious answer is that it may arise 
* For the fullest statement yet made of the habitats and years of appearance 
of the 14 broods of the 17-year race and the 7 broods of the 13-year race, see 
Bulletin 14, New Series, of the Division of Entomology, U.S. Department of Agri- 
culture, 1898. As an example of the overlapping of the habitats of two broods 
of the same race, observe that, on pp. 48 and 49 of this Bulletin, three of the 
counties of Iowa and three of Missouri are given as part of the district where 
Brood XIII will appear in the year 1912, and also as part of the district where 
Brood XIV will appear in 1913, both broods being of the 17-year race. Broods 
XXI and XXII, of the 17-year race, are also reported as appearing a year apart 
in Wilkes County, North Carolina. 
