368 THE HISTORY OF CREATION. 
dispersion of these species over a greater or less portion of 
the earth’s surface. But these migrations are also of great 
importance to the theory of development, because we can 
perceive in them a very important means for the origin of 
new species. When animals and plants migrate they meet in 
their new home, in the same way as do human emigrants, 
with conditions which are more or less different from those 
which they have inherited throughout generations, and to 
which they have been accustomed. The emigrants must 
either submit and adapt themselves to these new conditions 
of life or they perish. By adaptation their peculiar specific 
character becomes the more changed the greater the dif- 
ference between the new and the old home. The new 
climate, the new food, but above all, new neighbours in 
the forms of other animals and plants, influence and tend 
to modify the inherited character of the immigrant species, 
and if it is not hardy enough to resist the influences, then 
sooner or later a new species must arise out of it. In most 
eases this transformation of an immigrant species takes 
place so quickly under the influence of the altered struggle 
for life, that even after a few generations a new species 
arises from it. 
Migration has an especial influence in this way on all 
organisms with separate sexes. For in them the origin of 
new species by natural selection is always rendered difficult, 
or delayed, by the fact that the modified descendants oc- 
casionally again mix sexually with the unchanged original 
form, and thus by crossing return to the first form. But 
if such varieties have migrated, if great distances or 
barriers to migration—seas, mountains, etc.—have separated 
them from the old home, then the danger of a mingling 
