104 
DARWINISM 
CHAP. 
throughout all geological time ancl 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 extermina¬ 
tion 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, ojdy, which varied sufficiently in the right 
direction, the necessary modifications of structure or of func¬ 
tion would be brought about, just as surely as man has been 
able to breed the greyhound to hunt by sight and the fox¬ 
hound 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 characteristics of the 
changes in species that are likely to be effected, and how far 
they agree with what we observe in nature. 
