DIVERGENT EVOLUTION THROUGH SEGREGATION. 303 
pends 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, when those produced in a given district are 
more nearly related with each other than with those produced in sur- 
rounding 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 separated in space, it must 
always be re-enforced by other forms of segregation before well-defined 
species are produced that are capable of occupying the same district 
without inter-breeding. The vast majority of the divergent forms arising 
through local segregation are re-integrated with the surrounding forms, 
new divergences constantly coming in to take the place of the old; but 
if, during its brief period of local divergence, industrial or chronal 
segregation is introduced, the variety becomes more and more differen- 
tiated, and, as one after another the different forms of reflexive segre- 
gation arise, it passes into a well-defined species. There is however 
reason to believe that the order of events is often the reverse, reflexive 
forms of segregation being the cause of the first divergences. 
As spatial segregation does not depend upon diversity in the quali- 
ties and powers of the organism, so also it does not usually result in 
distributing the organism in different localities according to their dif- 
ferences of endowment. The causes that produce it are primarily 
separative, not segregative. 
Migration is produced by the natural powers of the organism, acting 
under the guidance of instincts that usually lead a group of individuals, 
rapable of propagating the species, to migrate together; while the 
organisms that are most dependent on activities in the environment for 
their distribution, are usually distributed in the form of seeds or germs, 
any one of which is capable of developing’ into a complete community. 
The causes of separation between the different sections, and of integra- 
tion between the members of one section, are therefore sufficiently 
