This fact is perfectly
compatible with numerous beings still retaining simple and but little
improved structures, fitted for simple conditions of life; it is likewise
compatible with some forms having retrograded in organisation, by having
become at each stage of descent better fitted for new and degraded habits
of life. Lastly, the wonderful law of the long endurance of allied forms
on the same continent--of marsupials in Australia, of edentata in America,
and other such cases--is intelligible, for within the same country the
existing and the extinct will be closely allied by descent.
Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the world to
another, owing to former climatical and geographical changes and to the
many occasional and unknown means of dispersal, then we can understand, on
the theory of descent with modification, most of the great leading facts in
Distribution. We can see why there should be so striking a parallelism in
the distribution of organic beings throughout space, and in their
geological succession throughout time; for in both cases the beings have
been connected by the bond of ordinary generation, and the means of
modification have been the same. We see the full meaning of the wonderful
fact, which has struck every traveller, namely, that on the same continent,
under the most diverse conditions, under heat and cold, on mountain and
lowland, on deserts and marshes, most of the inhabitants within each great
class are plainly related; for they are the descendants of the same
progenitors and early colonists.
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