The flock was allowed to wander at will and to interbreed freely.
One of the definitions of species is that they will not interbreed and propagate.
This will occur whenever they are of very different constitution; thus it is that widely differing species will not interbreed.
My experience tends to show that the two species will not interbreed.
Moreover, species which differ only in colour seem usually to interbreed in those parts where they meet.
Or will the two make peace, unite and interbreed, make war on us?
Notwithstanding their similarity they do not interbreed when brought together.
Of these, there are three sorts which keep apart from each other, and are thought not to interbreed.
This species will, however, interbreed with the domestic cat.
All mankind can interbreed freely, can learn to understand the same speech, can adapt itself to co-operation.