If we look at species as having been specially created, and at varieties as having been produced by secondary laws, this similarity would be an astonishing fact.

But it harmonises perfectly with the view that there is no essential distinction between species and varieties.

First crosses between forms, sufficiently distinct to be ranked as species, and their hybrids, are very generally, but not universally, sterile.

The sterility is of all degrees, and is often so slight that the most careful experimentalists have arrived at diametrically opposite conclusions in ranking forms by this test.