124 ANALYSIS OF THE POUR PRINCIPLES (CONTINUED). 



l< -til separation, or if during the period of 221 years of cyclical separa- 

 tism 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 rigidlv 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 was as 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. 



.Si (i\o)itil 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, 

 in a 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 [3-year race, see 

 Bulletin 14, New Series, of the Division of Entomology, U.S. Department of Agri- 

 culture, [898. As an example of the overlapping of the habitats of two broods 

 of the same race, observe that, on pp. 4S 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. 



