ustralia 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.

Having then all the facts of animal natural history at his disposal, and
the uniform principles in each body of fact that demonstrate evolution, it
is small wonder that the evolutionist seems to dogmatize when he asserts
that descent with adaptive and divergent modification is true for all
species of living things. The case is complete as it stands to-day, while
it is even more significant that every new discovery falls into line with
what is already known, and takes its natural place in the all-inclusive
doctrine of organic evolution. Because this explanation of the
characteristics of the living world is more reasonable than any other,
science teaches that it is true.

IV

EVOLUTION AS A NATURAL PROCESS

The purpose of the discussions up to this point has been to present the
reasons drawn from the principal classes of zooelogical facts for believing
that living things have transformed naturally to become what they now are.
Even if it were possible to make an exhaustive analysis of all of the
known phenomena of animal structure, development, and fossil succession,
the complete bodies of knowledge could not make the evolutionary
explanation more real and evident than it is shown to be by the simple
facts and principles selected to constitute the foregoing outline. We have
dealt solely with the evidences as to the fact of evolution; and now,
having assured ourselves that it is worth while to so do, we may turn to
the intelligible