302 DIVERGENT EVOLUTION THROUGH SiHGREGATION. 
(c) Spatial segregation 
is Segregation arising from the relations in which the organism stands 
to space. \ 
I distinguish two forms, viz: geographical and local segregation. 
Geographical segregation is segregation that arises from the distribu- 
tion of the species in districts separated by geographical barriers that 
prevent free inter-breeding. Decided differences of climate in neigh- 
boring districts and regions may be classed as geographical barriers. 
Local segregation is segregation 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 subdi- 
vided 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 occu- 
pying the different parts of the area. Though it is often difficult to 
say whether a given case of segregation should be classed as geograph- 
ical or local, still the distinction will be found useful, for the results 
will differ according as the segregation 1s chiefly due to barriers or to 
wide diffusion of the species. In geographical segregation 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 sur- 
rounding localities by individuals presenting every shade of interme: 
diate character; and in general terms it may be said that the forms 
most widely separated in space are most widely divergent in charac- 
ter. It is of course apparent that when the divergence has reached a 
certain point, the differentiated forms may occupy the same districts 
without inter-breeding, for they will be kept apart by some, if not all, 
of the different forms of industrial, chronal, conjunctional, and impreg- 
national segregation. 
Three different forms of spatial segregation may be distinguished 
according to the causes by which they are produced, viz: 
6. Migrational segregation, caused by powers of locomotion in the 
organism. 
7. Transportational segregation, caused by activities in the environ- 
ment 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). 
8. Geological segregation, caused by geological changes dividing the 
territory occupied by a species into two or more sections. For exam- 
ple, 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 activities 
in the environment; and though the distribution of every species de- 
