354 
GEOGRAPHICAL BISTRIBUTIOK. 
Chap, XI. 
merly continuous range of many species. So that 
we are reduced to consider whether the exceptions to 
continuity of range are so numerous and of so grave a 
nature, that we ought to give up the belief, rendered 
probable by general considerations, that each species 
has been produced within one area, and has migrated 
thence as far as it could. It would be hopelessly tedious 
to discuss all the exceptional cases of the same species, 
now’^ living at distant and separated points; nor do I 
for a moment pretend that any explanation could be 
offered of many such cases. But after some preliminary 
remarks, I will discuss a few of the most striking classes 
of facts; namely, the existence of the same species on 
the summits of distant mountain-ranges, and at distant 
points in the arctic and antarctic regions; and secoudly 
(in the following chapter), the wide distribution of fresh¬ 
water productions ; and thirdly, the occurrence of the 
same terrestrial species on islands and on the mainland, 
though separated by hundreds of miles of open sea. If 
the existence of the same , species at distant and isolated 
points of the earth’s surface, can in many instances be 
explained on the view of each species having migrated 
from a single birthplace; then, considering our ignor¬ 
ance with respect to former climatal and geographical 
changes and various occasional means of transport, the 
belief that this has been the universal law, seems to me 
incomparably the safest. 
In discussing this subject, we shall be enabled at the 
same time to consider a point equally important for us, 
namely, whether the several distinct species of a genus, 
w^hich on my theory have all descended from a common 
progenitor, can have migrated (undergoing modification 
during some part of their migration) from the area 
inhabited by their progenitor. If it can be shown to 
be almost invariably the case, that a region, of which 
