CHRONAL ISOLATION. 125 



under some special influence of the environment upon members of the 

 species that are geographically or locally isolated from the rest of the 

 species. 



But may not variation in the flowering season of a fairly homogen- 

 eous species 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 com- 

 patible with the best adaptation to the environment? I believe that 

 it not only may, but must have this effect ; and that the result will be 

 that the average form which flowers at the most favorable 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 propagation, 

 this principle is segregative, but in other respects it is simply separa- 

 tive, unless through the principle of correlated variation other charac- 

 ters are directly connected with the constitution that determines the 

 season. It will be observed that seasonal isolation is produced by a 

 parallel and simultaneous change in the constitution of members in 

 one place sufficient to propagate the species; while cyclical segre- 

 gation is produced by a simultaneous acceleration or retardation in 

 the development of members in one place sufficient to propagate the 

 species without disturbing the regular action of the constitution under 

 ordinary circumstances. 



5. Spatial Isolation. 



Spatial isolation is isolation arising from the relations in which the 

 organism stands to space. I distinguish two forms, viz, geographical 

 and local isolation. 



Geographical isolation is isolation that arises from the distribution of 

 the species in districts separated by geographical barriers that prevent 

 free interbreeding. Decided differences of climate in neighboring 

 districts may be classed as geographical barriers. 



Local isolation is isolation that arises when a species with small 

 powers of migration and small opportunities for transportation has 

 been, in time, very widely distributed over an area that is not sub- 

 divided by geographical barriers. The segregation in this case is 

 due to the disproportion between the size of the area occupied and the 

 powers of communication existing between the members of the 

 species occupying the different parts of the area. Though it is often 

 difficult to say whether a given case of isolation should be classed as 

 geographical or local, still the distinction will be found useful, for the 



