I am far from supposing that all the difficulties in regard to the distribution and affinities of the identical and allied species, which now live so widely separated in the north and south, and sometimes on the intermediate mountain ranges, are removed on the views above given.

The exact lines of migration cannot be indicated.

We cannot say why certain species and not others have migrated; why certain species have been modified and have given rise to new forms, while others have remained unaltered.