A broken or interrupted range may often be accounted for by the extinction of the species in the intermediate regions.

It cannot be denied that we are as yet very ignorant as to the full extent of the various climatical and geographical changes which have affected the earth during modern periods; and such changes will often have facilitated migration.

As an example, I have attempted to show how potent has been the influence of the Glacial period on the distribution of the same and of allied species throughout the world.