ally, under changed
conditions, bring about whatever change of structure or habits may be
necessitated by them. The changed conditions to which we refer are such
as we know have occurred throughout all geological time and in every
part of the world. Land and water have been continually shifting their
positions; some regions are undergoing subsidence with diminution of
area, others elevation with extension of area; dry land has been
converted into marshes, while marshes have been drained or have even
been elevated into plateaux. Climate too has changed again and again,
either through the elevation of mountains in high latitudes leading to
the accumulation of snow and ice, or by a change in the direction of
winds and ocean currents produced by the subsidence or elevation of
lands which connected continents and divided oceans. Again, along with
all these changes have come not less important changes in the
distribution of species. Vegetation has been greatly modified by changes
of climate and of altitude; while every union of lands before separated
has led to extensive migrations of animals into new countries,
disturbing the balance that before existed among its forms of life,
leading to the extermination of some species and the increase of others.

When such physical changes as these have taken place, it is evident that
many species must either become modified or cease to exist. When the
vegetation has changed in character the herbivorous animals must become
able to live on new and perhaps less nutritious food; while the change
from a damp to a dry climate may necessitate migration at certain
periods to escape destruction by drought. This will expose the species
to new dangers, and require special modifications of structure to meet
them. Greater swiftness, increased cunning, nocturnal habits, change of
colour, or the power of climbing trees and living for a time on their
foliage or fruit, may be the means adopted by different species to bring
themselves into harmony with the new conditions; and by the continued
survival of those individuals, only, which varied sufficiently in the
right direction, the necessary modifications of structure or of function
would be brought about, just as surely as man has been able to breed the
greyhound to hunt by sight and the foxhound by scent, or has produced
from the same wild plant such distinct forms as the cauliflower and the
brussels sprouts.

We will now consider the special characte