THROUGH CUMULATIVE SEGREGATION. 
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water, and tlie action of migratory species upon those that can 
simply cling). 
8. Geological Segregation, caused by geological changes di- 
viding the territory 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 cannot 
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 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 Segregation 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 whenever extraordinary dispersal -plants a colony beyond the 
range of ordinary dispersal. In other words, w T hen those pro- 
duced in a given district are more nearly related with each other 
than with those produced in surrounding districts, there local 
segregation has existed. 
There is one important respect in which Spatial Segregation 
differs from all other forms of Environal Segregation, namely, in 
its ordinary operation it does not depend directly upon diversity 
in the qualities and powers of the organism. The dispersion of 
the members of a species would not be prevented if each was 
exactly like every other ; though, of course, if there were no 
power of variation, separate breeding would have no influence in 
producing divergence of character. It follows that every species 
is, or is more or less liable to be, affected by Spatial Segregation ; 
and it often happens that other forms of Segregation arise 
through the previous operation of this form ; but as Spatial 
Segregation prevents organisms from crossing only when sepa- 
rated in space, it must always be reinforced by other forms 
of segregation before well-defined species are produced that 
are capable of occupying the same district without interbreeding. 
The vast majority of the divergent forms arising through Local 
Segregation are reintegrated with the surrounding forms, new 
divergences constantly coming in to take the place of the old ; 
