America, though in none of
these cases are the animals natives of the countries in which they
thrive so well. And lastly, in illustration of the fact that allied
forms are not always found in adjacent regions, we have the tapirs,
which are found only on opposite sides of the globe, in tropical America
and the Malayan Islands; the camels of the Asiatic deserts, whose
nearest allies are the llamas and alpacas of the Andes; and the
marsupials, only found in Australia and on the opposite side of the
globe, in America. Yet, again, although mammalia may be said to be
universally distributed over the globe, being found abundantly on all
the continents and on a great many of the larger islands, yet they are
entirely wanting in New Zealand, and in a considerable number of other
islands which are, nevertheless, perfectly able to support them when
introduced.

Now most of these difficulties can be solved by means of well-known
geographical and geological facts. When the productions of remote
countries resemble each other, there is almost always continuity of land
with similarity of climate between them. When adjacent countries differ
greatly in their productions, we find them separated by a sea or strait
whose great depth is an indication of its antiquity or permanence. When
a group of animals inhabits two countries or regions separated by wide
oceans, it is found that in past geological times the same group was
much more widely distributed, and may have reached the countries it
inhabits from an intermediate region in which it is now extinct. We
know, also, that countries now united by land were divided by arms of
the sea at a not very remote epoch; while there is good reason to
believe that others now entirely isolated by a broad expanse of sea were
formerly united and formed a single land area. There is also another
important factor to be taken account of in considering how animals and
plants have acquired their present peculiarities of
distribution,--changes of climate. We know that quite recently a glacial
epoch extended over much of what are now the temperate regions of the
northern hemisphere, and that consequently the organisms which inhabit
those parts must be, comparatively speaking, recent immigrants from more
southern lands. But it is a yet more important fact that, down to middle
Tertiary times at all events, an equable temperate climate, with a
luxuriant vegetation, extended to far within the arctic circle, over
wha