ing 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 an ignorance of the
conditions above alluded to. My own personal observation and experience
in these matters have led me to the conviction that every geological
period has had its own representatives, and that no single species has
been repeated in successive ages.

The laws regulating the geographical distribution of animals, and their
combination into distinct zooelogical provinces called faunae, with
definite limits, are very imperfectly understood as yet; but so closely
are all things linked together from the beginning till to-day, that I am
convinced we shall never find the clew to their meaning till we carry on
our investigations in the past and the present simultaneously. The same
principle according to which animal and vegetable life is distributed
over the surface of the earth now, prevailed in the earliest geological
periods. The geological deposits of all times have had their
characteristic faunae under various zones, their zooelogical provinces
presenting special combinations of animal and vegetable life over
certain regions, and their representative types reproducing in different
countries, but under similar latitudes, the same groups with specific
differences.

Of course, the nearer we approach the beginning of organic life, the
less marked do we find the differences to be; and for a very obvious
reason. The inequalities of the earth's surface, her mountain-barriers
protecting whole continents from the Arctic winds, her open plains
exposing others to the full force of the polar blasts, her snug valleys
and her lofty heights, her tablelands and rolling prairies, her
river-systems and her dry deserts, her cold ocean-currents pouring down
from the high North on some of her shores, while warm ones from tropical
seas carry their softer influence to others,--in short, all the
contrasts in the external configuration of the globe, with the physical
conditions attendant upon them, are naturally accompanied by a
corresponding variety in animal and vegetable life.

But in the Silurian age, when there were no elevations higher than the
Canadian hills, when water covered the f