Pigeon fanciers know that when they have once obtained, by crossbreeding and selection, a particular form or feather, the utmost care is needed to preserve it. If the parents are not selected the progeny wilt gradually revert towards the original wild pigeon type.
It should be borne in mind that what we call a species as distinct from a mere variety, is a more or less arbitrary or provisional thing dependent on the state of science for the time. Species are constantly being lumped together by some and separated by others. It follows most probably, that while some species are really types—i.e., one can never pass into the other and lose its essentials, unless it is destined to disappear (like the pterodactyle), not being wanted in the whole scheme—other species are really only varieties, and maybe lost or modified without limit.
We may well regard the mule as a peculiar form just such as the evolutionist would rejoice to see: here is a modified species, which has qualities different from those of either of the parent stock, and well fitted "to struggle for existence." Yet this modified race would, if left to itself, die out.