340 
GEOLOGICAL SUCCESSIOJ^. 
Chap. X. 
and, on the other hand, by similarity of conditions, for 
the uniformity of the same types in each during the 
later tertiary periods. Nor can it be pretended that it 
is an immutable law that marsupials should have been 
chiefly or solely produced in Australia; or that Eden¬ 
tata and other American types should have been solely 
produced in South America. For we know that Europe 
in ancient times was peopled by numerous marsupials; 
and I have shown in the publications above alluded to, 
that in America the law of distribution of terrestrial 
mammals was formerly different from what it now is. 
North America formerly partook strongly of the pre¬ 
sent character of the southern half of the continent; 
and the southern half was formerly more closely allied, 
than it is at present, to the northern half. In a similar 
manner we knOw from Falconer and Cautley’s dis¬ 
coveries, that northern India was formerly more closely 
related in its mammals to Africa than it is at the pre¬ 
sent time. Analogous facts could be given in relation 
to the distribution of marine animals. 
On the theory of descent with modification, the great 
law of the long enduring, but not immutable, succession 
of the same types within the same areas, is at once 
explained; for the inhabitants of each quarter of the 
world will obviously tend to leave in that quarter, 
during the next succeeding period of time, closely 
allied though in some degree modified descendants. If 
the inhabitants of one continent formerly differed 
greatly from those of another continent, so will their 
modified descendants still difi“er in nearly the same 
manner and degree. But after very long intervals of 
time and after great geographical changes, permitting 
much inter-migration, the feebler will yield to the 
more dominant forms, and there will be nothing im¬ 
mutable in the laws of past and present distribution. 
