OF ZOOLOGICAL FOCI. ^47 
turkey among birds ; and for these two classes of animals 
it is probably correctly defined ; but for the terrestrial 
mollusks, some other division is necessary ; for the most 
■widely diffused among them does not occupy so large a 
space, and several groups of species are much more lim- 
ited, as we have endeavored to show. It will be objected 
to this theory of distinct zoological centres, that it can- 
not be natural, because it concerns only one department 
of animals ; and that, as nature acts through general laws, 
these separate plans of diffusion are inconsistent with 
this principle. The objection, in our view, has but little 
weight ; the laws of nature are known only by their 
results, and as we see that different classes of animals 
have different capacities and different powers of action, 
and of resisting action, we must believe that they were 
formed to be subject to different influences ; and that 
their diffusion may, without any inconsistency of purpose, 
have been regulated by different plans. And the gen- 
erally received opinion, of the successive creation of the 
different classes of animals, after intervals of greater or 
less durations, adds strong confirmation to our theory ; 
for, admitting this to be true, the centres from which 
sprung the various mollusks were established, and the 
causes influencing their extension and distribution were 
in operation, for an indefinite period in advance of even 
the existence of the animals of the higher classes. 
There are difficulties which it seems to us can hardly 
be overcome on any other supposition. If all the species 
had been created at one time and at one place, they 
