have proceeded from some one source;
then I think all the grand leading facts of geographical distribution
are explicable on the theory of migration... together with subsequent
modification and the multiplication of new forms." We can thus
understand how it is that in some countries the inhabitants "are linked
to the extinct beings which formerly inhabited the same continent."
We can see why two areas, having nearly the same physical conditions,
should often be inhabited by very different forms of life,... and "we can
see why in two areas, however distant from each other, there should be a
correlation, in the presence of identical species... and of distinct but
representative species." ("The Origin of Species" (1st edition), pages
408, 409.)

Darwin's reluctance to assume great geological changes, such as a
land-connection of Europe with North America, is easily explained by the
fact that he restricted himself to the distribution of the present and
comparatively recent species. "I do not believe that it will ever be
proved that within the recent period continents which are now quite
separate, have been continuously, or almost continuously, united with
each other, and with the many existing oceanic islands." (Ibid. page
357.) Again, "believing... that our continents have long remained in
nearly the same relative position, though subjected to large, but
partial oscillations of level," that means to say within the period of
existing species, or "within the recent period." (Ibid. page. 370.) The
difficulty was to a great extent one of his own making. Whilst almost
everybody else believed in the immutability of the species, which
implies an enormous age, logically since the dawn of creation, to him
the actually existing species as the latest results of evolution, were
necessarily something very new, so young that only the very latest of
the geological epochs could have affected them. It has since come to
our knowledge that a great number of terrestrial "recent" species, even
those of the higher classes of Vertebrates, date much farther back than
had been thought possible. Many of them reach well into the Miocene, a
time since which the world seems to have assumed the main outlines of
the present continents.

In the year 1866 appeared A. Murray's work on the "Geographical
Distribution of Mammals", a book which has perhaps received less
recognition than it deserves. His treatment of the general introductory
questions marks