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.