have
characterized the successive epochs is often understood as indicating a
difference of another kind than that which distinguishes animals now
living in different parts of the world. This is a mistake. They are
so-called representative types all over the globe, united to each other
by structural relations and separated by specific differences 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 some 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 appreciat