126 ANALYSIS OF THE FOUR PRINCIPLES (CONTINUED). 



results will differ according as the isolation is chiefly due to barriers or 

 to wide diffusion of the species. In geographical isolation the result 

 is usually the development of well-defined varieties or species on oppo- 

 site sides of the barriers; but in local segregation it often happens 

 that the forms found in any given locality are connected with those 

 in surrounding localities by individuals presenting every shade of 

 intermediate character, and in general terms it may be said that the 

 forms most widely separated in space are most widely divergent in 

 character. It is, of course, apparent that when the divergence has 

 reached a certain point the differentiated forms may occupy the same 

 districts without interbreeding, for they will be kept apart by some, 

 if not all, of the different forms of autonomic isolation. 



Three different forms of spatial segregation may be distinguished 

 according to the causes by which they are produced, viz: 



Migrational isolation, caused by powers of locomotion in the or- 

 ganism. 



Trans portational isolation, caused by activities in the environment 

 that distribute the organism in different districts. Prominent among 

 these are currents of atmosphere and of water, and the action of 

 migratory species upon those that can simply cling. 



Geological isolation, caused by geological changes dividing the ter- 

 ritory occupied by a species into two or more sections. For example, 

 geological subsidence may divide the continuous area occupied by a 

 species into several islands, separated by channels which the creatures 

 in epiestion can not pass. 



Migration differs from transportation simply in that the former is 

 the direct result of activities in the organism, and the latter of activi- 

 ties in the environment, and though the distribution of every species 

 depends on the combined action of both classes of activities, it is 

 usually easy to determine to which class the carrying pow T er belongs. 

 The qualities of the thistle-down enable it to float in the air, but it is 

 the wind that carries it afar. 



Some degree of local isolation exists whenever the members of a 

 species produced in a given area are more likely to interbreed with 

 each other than with those produced in surrounding areas, or when- 

 ever extraordinary dispersal plants a colony beyond the range of ordi- 

 nary dispersal — in other words, when those produced in a given dis- 

 trict are more nearly related with each other than with those produced 

 in surrounding districts, there local isolation has existed. 



There is one important respect in which spatial isolation differs from 

 all other forms of isolation, namely: In its ordinary operation it does 



