Chap. XII. 
SUMMABY., 
409 
loped in great force^ some existing in scanty numbers— 
in the dififerent great geographical provinces of the 
world. 
On these same principles, we can understand, as I 
have endeavoured to show, why oceanic islands should 
have few inhabitants, but of these a great number 
should be endemic or peculiar; and why, in relation to 
the means of migration, one group of beings, even within 
the same class, should have all its species endemic, and 
another group should have all its species common to 
other quarters of the world. We can see why whole 
gTOups of organisms, as batrachians and terrestrial mam¬ 
mals, should be absent from oceanic islands, whilst the 
most isolated islands possess their own peculiar species of 
aerial mammals or bats. We can see why there should 
be some relation between the presence of mammals, in 
a more or less modified condition, and the depth of 
the sea between an island and the mainland. We can 
clearly see why all the inhabitants of an archipelago, 
though specifically distinct on the several islets, should 
be closely related to each other, and likewise be related, 
but less closely, to those of the nearest continent or 
other source whence immigrants w^ere probably derived. 
We can see why in two areas, however distant from each 
other, there should be a correlation, in the presence of 
identical species, of varieties, of doubtful species, and of 
distinct but representative species. 
As the late Edward Eorbes often insisted, there is a 
striking parallelism in the laws of life throughout time 
and space: the laws governing the succession of forms in 
past times being nearly the same with those governing 
at the present time the differences in different areas. 
We see this in many facts. The endurance of each 
species and group of species is continuous in time ; for 
the exceptions to the rule are so few, that they may 
T 
