244 Darwin, and after Darwin. 
correlation between the birth of new species and the 
immediate pre-existence of closely allied species on 
the same area—or, at most, on closely contiguous areas. 
Where a continuous area has long been circum- 
scribed by barriers of any kind, which prevent the 
animals from wandering beyond it, then we find that 
all the species, both extinct and living, constitute 
more or less a world of their own; while, on the 
other hand, where the animals are free to migrate 
from one area to another, the course of their migra- 
tions is marked by the origination of new species 
springing up ez rozze, and serving to connect the 
older, or metropolitan, forms with the younger, or 
colonising, forms in the way of a graduated series. 
This principle, however, admits of being traced only 
in certain cases of species belonging to the same 
genus, of genera belonging to the same family, or, 
at most, of families belonging to the same order. 
In other words, the more general the structural 
affinity, the more general is the geographical ex- 
tension—as we should expect to be the case on the 
theory of descent with branching modifications, seeing 
that the larger, the older, and the more diverse the 
group of organisms compared, the greater must be 
their chances of dispersal. 
These general considerations led us to contemplate 
more in detail the correlation between structural 
affinity and barriers to free migration. Such barriers, 
of course, differ in the cases of different organisms. 
Matine organisms are stopped by land, unsuitable 
temperature, or unsuitable depths; fresh-water or- 
ganisms by sea and by mountain-chains ; terrestrial 
organisms chiefly by water. Now it is a matter of 
