nces of
the same kind as those that unite and separate animals of different
geological periods. Take, for instance, mud-flats or sandy shores in the
same latitudes of Europe and America; we find living on each animals of
the same structural character and of the same general appearance,
but with certain specific differences, as of color, size, external
appendages, etc. They represent each other on the two continents.
The American wolves, foxes, bears, rabbits, are not the same as the
European, but those of one continent are as true to their respective
types as those of the other; under a somewhat different aspect they
represent the same groups of animals. In certain latitudes, or under
conditions of nearer proximity, these differences may be less marked.
It is well known that there is a great monotony of type, not only among
animals and plants, but in the human races also, throughout the Arctic
regions; and the animals characteristic of the high North reappear under
such identical forms in the neighborhood of the snow-fields in lofty
mountains, that to trace the difference between the ptarmigans, rabbits,
and other gnawing animals of the Alps, for instance, and those of the
Arctics, is among the most difficult problems of modern science.

And so is it also with the animated world of past ages; in similar
deposits of sand, mud, or lime, in adjoining regions of the same
geological age, identical remains of animals and plants may be
found, while at greater distances, but under similar circumstances,
representative species may occur. In very remote regions, however,
whether the circumstances be similar or dissimilar, the general aspect
of the organic world differs greatly, remoteness in space being thus in
some measure an indication of the degree of affinity between different
faunae. In deposits of different geological periods immediately
following each other we sometimes find remains of animals and plants so
closely allied to those of earlier or later periods that at first sight
the specific differences are hardly discernible. The difficulty of
solving these questions, and of appreciating correctly the differences
and similarities between such closely allied organisms, explains the
antagonistic views of many naturalists respecting the range of existence
of animals, during longer or shorter geological periods; and the
superficial way in which discussions concerning the transition of
species are carried on is mainly owing to