The broad and consistent principle underlying these and related facts is
this: _there is a general correspondence between the differences displayed
by the organisms of two regions and the degree of isolation or proximity
of these two areas_. Thus the disconnected but neighboring areas of the
Galapagos Islands and South America support species that resemble each
other closely, for the reasons given before; long isolated areas like
Australia and its surroundings possess peculiar creatures like the
egg-laying mammals, and all of the pouched animals or marsupials with only
one or two exceptions like our own American opossum,--a correlation
between a geological and geographical discontinuity on the one hand and a
peculiarity on the other that reinforces our confidence in the
faunal evolutionary interpretation of the facts of distribution.
It is true that the various classes of animals do not always appear with
coextensive ranges. The barriers between two groups of related species
will not be the same in all cases. A range like the Rocky Mountains will
keep fresh-water fish apart, while birds and mammals can get across
somewhere at some time. All these things must be taken into account in
analyzing the phenomena of distribution, and many factors must be given
due attention; but in all cases the reasons for the particular state of
affairs in geographical and biological respects possess an evolutionary
significance.
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