e. There 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 it is 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 o