usly and
continued in perfect health and vigour, although all derived from a
single pair. But in all these cases there has been rigid selection by
which the weak or the infertile have been eliminated, and with such
selection there is no doubt that the ill effects of close interbreeding
can be prevented for a long time; but this by no means proves that no
ill effects are produced. Mr. Huth himself quotes M. Allie, M. Aube,
Stephens, Giblett, Sir John Sebright, Youatt, Druce, Lord Weston, and
other eminent breeders, as finding from experience that close
interbreeding _does_ produce bad effects; and it cannot be supposed that
there would be such a consensus of opinion on this point if the evil
were altogether imaginary. Mr. Huth argues, that the evil results which
do occur do not depend on the close interbreeding itself, but on the
tendency it has to perpetuate any constitutional weakness or other
hereditary taints; and he attempts to prove this by the argument that
"if crosses act by virtue of being a cross, and not by virtue of
removing an hereditary taint, then the greater the difference between
the two animals crossed the more beneficial will that act be." He then
shows that, the wider the difference the less is the benefit, and
concludes that a cross, as such, has no beneficial effect. 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 bef