" 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.
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