Chap. XIV. 
BECAPITULATION. 
477 
modification liave been the same. We see the full 
meaning of the wonderful fact, which must have struck 
every traveller, namely, that on the same continent, 
under the most diverse conditions, under heat and cold, 
on mountain and lowland, on deserts and marshes, most 
of the inhabitants within each great class are plainly 
related; for they will generally be descendants of the 
same progenitors and early colonists. On this same 
principle of former migration, combined in most cases 
with modification, we can understand, by the aid of the 
Glacial period, the identity of some few plants, and the 
close alliance of many others, on the most distant moun¬ 
tains, under the most different climates; and hkewise 
the close alliance of some of the inhabitants of the sea 
in the northern and southern temperate zones, though 
separated by the whole intertropical ocean. Although 
two areas may present the same physical conditions of 
life, we need feel no surprise at their inhabitants being 
widely different, if they have been for a long period 
completely separated from each other; for as the rela¬ 
tion of organism to organism is the most important of 
all relations, and as the two areas will have received 
colonists from some third source or from each other, at 
various periods and in different proportions, the course 
of modification in the two areas will inevitably be 
different. 
On this view of migration, with subsequent modifica¬ 
tion, we can see why oceanic islands should be inhabited 
by few species, but of these, that many should be 
peculiar. We can clearly see why those animals which 
cannot cross wide spaces of ocean, as frogs and terrestrial 
mammals, should not inhabit oceanic islands; and why, 
on the other hand, new and peculiar species of bats, 
which can traverse the ocean, should so often be found 
on islands far distant from any continent. Such facts 
