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.

We cannot hope to explain such facts, until we can say why one species and not another becomes naturalised by man's agency in a foreign land; why one species ranges twice or thrice as far, and is twice or thrice as common, as another species within their own homes.