THEOUaH CUMDLATIVE SEaEEGATION. 



229 



septemdecim being entirely separated from all other broods of the 

 same race by being belated a year, may be modified by forms o£ 

 Natural Selection that never arise in tliese other broods. And 

 this may be the case even if a brood observing the ordinary time 

 is always associated with it in locality. 



5. Seasonal Segregation is produced whenever the season for 

 reproduction in any section of the species is such that it cannot 

 interbreed with other sections of the species. It needs no argu- 

 ment to show that if, in a species of plant 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 circum- 

 stances are varieties of this kind likely to arise ? Is a casual 

 sport of this kind likely to transmit to subsequent generations a 

 permanently changed constitution ? If not, how is the new con- 

 stitution acquired? One obyious answer is that it may arise 

 under some special influence of the environment upon members 

 of the species that are geographically or locally segregated from 

 the rest of the species. 



But may not the variation in the season of flowering be the 

 cause of segregation that will directly tend to produce greater 

 variation in that respect in the next generation, and so on till 

 the divergence in the constitutional adaptation to season is 

 carried to the greatest extreme that is compatible with the en- 

 vironment ? I believe that it not only may, but must have that 

 effect; but we should remember that the average form which 

 flowers at the height of the season will so vastly predominate 

 over the extreme forms that the latter will be but stragglers in 

 comparison. 



In regard to the one point of the season of readiness for pro- 

 pagation, this principle is segregative ; but in other respects it is 

 simply separative, unless through the principle of correlated 

 variation other characters are directly connected with the con- 

 stitution that determines the season. It will be observed that 

 Seasonal Segregation is produced by a parallel and simultaneous 

 change in the constitution of members in one place sufiicient to 

 propagate the species ; while Cyclical Segregation is produced 

 by a simultaneous acceleration or retardation iu the development 

 of members in one place suflicient to propagate the species with- 

 out disturbing the regular action of the constitution under 

 ordinary circumstances. 



