ceans, and with only such changes of sea
and land as are indicated by the extent of the comparatively shallow
seas surrounding and connecting the existing continents, we are able to
account for the anomaly of allied forms occurring only in remote and
widely separated areas. These examples really constitute crucial tests,
because, of all classes of animals, mammalia are least able to surmount
physical barriers. They are obviously unable to pass over wide arms of
the sea, while the necessity for constant supplies of food and water
renders sandy deserts or snow-clad plains equally impassable. Then,
again, the peculiar kinds of food on which alone many of them can
subsist, and their liability to the attacks of other animals, put a
further check upon their migrations. In these respects almost all other
organisms have great advantages over mammals. Birds can often fly long
distances, and can thus cross arms of the sea, deserts, or mountain
ranges; insects not only fly, but are frequently carried great distances
by gales of wind, as shown by the numerous cases of their visits to
ships hundreds of miles from land. Reptiles, though slow of movement,
have advantages in their greater capacity for enduring hunger or thirst,
their power of resisting cold or drought in a state of torpidity, and
they have also some facilities for migration across the sea by means of
their eggs, which may be conveyed in crevices of timber or among masses
of floating vegetable matter. And when we come to the vegetable kingdom,
the means of transport are at their maximum, numbers of seeds having
special adaptations for being carried by mammalia or birds, and for
floating in the water, or through the air, while many are so small and
so light that there is practically no limit to the distances they may be
carried by gales and hurricanes.

We may, therefore, feel quite certain that the means of distribution
that have enabled the larger mammalia to reach the most remote regions
from a common starting-point, will be at least as efficacious, and
usually far more efficacious, with all other land animals and plants;
and if in every case the existing distribution of this class can be
explained on the theory of oceanic and continental permanence, with the
limited changes of sea and land already referred to, no valid objections
can be taken against this theory founded on anomalies of distribution in
other orders. Yet nothing is more common than for students of this