nd one has also been
found in North-western India, but none whatever among all the rich deposits
of mammalia in Europe. We are thus told, as clearly as possible, that from
the North American continent as a centre the camel tribe spread westward,
over now-submerged land at the shallow Behring Straits and Kamschatka Sea,
into Asia, and southward along the Andes into South America. Tapirs are
even more interesting and instructive. Their remotest known ancestors
appear in Western Europe in the early portion of the Eocene period; in the
latter Eocene and the Miocene other forms occur both in Europe and North
America. These seem to have become extinct in North America, while in
Europe they developed largely into many forms of true tapirs, which at a
much later period found their way again to North, and thence to South,
America, where their remains are found in caves and gravel deposits. It is
an instructive fact that in the Eastern continent, where they were once so
abundant, they have dwindled down to a single species, existing in small
numbers in the Malay Peninsula, Sumatra, and Borneo only; while in the
Western continent, where they are comparatively recent immigrants, they
occupy a much larger area, and are represented by three or four distinct
species. Who could possibly have imagined such migrations, and extinctions,
and changes of distribution as are demonstrated in the case of the tapirs,
if we had only the distribution of the existing species to found an opinion
upon? Such cases as these--and there are many others equally striking--show
us with the greatest distinctness how nature has worked in bringing about
the examples of anomalous distribution that everywhere meet us; and we
must, on every ground of philosophy and common sense, apply the same method
of interpretation to the more numerous instances of anomalous distribution
we discover among such groups as reptiles, birds, and insects, where we
rarely have any direct evidence of their past migrations through the
discovery of {422} fossil remains. Whenever we can trace the past history
of any group of terrestrial animals, we invariably find that its actual
distribution can be explained by migrations effected by means of
comparatively slight modifications of our existing continents. In no single
case have we any direct evidence that the distribution of land and sea has
been radically changed during the whole lapse of the Tertiary and Secondary
periods, while, as we ha