I have said a large genus, because as we saw in the second chapter, on an average more species vary in large genera than in small genera; and the varying species of the large genera present a greater number of varieties.

We have, also, seen that the species, which are the commonest and most widely-diffused, vary more than do the rare and restricted species.

Let (A) be a common, widely-diffused, and varying species, belonging to a genus large in its own country.