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 tsolation, 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 question 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 power 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 
