A parallel argument would be, that change of air, as from inland to the sea-coast, or from a low to an elevated site, is not beneficial in itself, because, if so, a change to the tropics or to the polar regions should be more beneficial.

In both these cases it may well be that no benefit would accrue to a person in perfect health; but then there is no such thing as "perfect health" in man, and probably no such thing as absolute freedom from constitutional taint in animals.

The experiments of Mr.
Darwin, showing the great and immediate good effects of a cross between distinct strains in plants, cannot be explained away; neither can the innumerable arrangements to secure cross-fertilisation by insects, the real use and purport of which will be discussed in our eleventh chapter.
On the whole, then, the evidence at our command proves that, whatever may be its ultimate cause, close interbreeding _does_ usually produce bad results; and it is only by the most rigid selection, whether natural or artificial, that the danger can be altogether obviated.
_Fertile Hybrids among Animals._ One or two more cases of fertile hybrids may be given before we pass on to the corresponding experiments in plants.

Professor Alfred Newton received from a friend a pair of hybrid ducks, bred from a common duck (Anas boschas), and a pintail (Dafila acuta).

From these he obtained four ducklings, but these latter, when grown up, proved infertile, and did not breed again.