a long period with impunity by the selection of the most vigorous
and healthy individuals; but sooner or later evil follows. The evil,
however, comes on so slowly and gradually that it easily escapes
observation, but can be recognised by the almost instantaneous manner in
which size, constitutional vigour, and fertility are regained when animals
that have long been interbred are crossed with a distinct family.

These two great classes of facts, namely, the good derived from crossing,
and the evil from close interbreeding, with the consideration of the
innumerable adaptations throughout nature for compelling, or favouring, or
at least permitting, the occasional union of distinct individuals, taken
together, lead to the conclusion that it is a law of nature that organic
beings shall not fertilise themselves for perpetuity. This law was first
plainly hinted at in 1799, with respect to plants, by Andrew Knight,[440]
and, not long afterwards, that sagacious observer Koelreuter, after showing
how well the Malvaceae are adapted for {176} crossing, asks, "an id aliquid
in recessu habeat, quod hujuscemodi flores nunquam proprio suo pulvere, sed
semper eo aliarum suae speciei impregnentur, merito quaeritur? Certe natura
nil facit frustra." Although we may demur to Koelreuter's saying that
nature does nothing in vain, seeing how many organic beings retain
rudimentary and useless organs, yet undoubtedly the argument from the
innumerable contrivances, which favour the crossing of distinct individuals
of the same species, is of the greatest weight. The most important result
of this law is that it leads to uniformity of character in the individuals
of the same species. In the case of certain hermaphrodites, which probably
intercross only at long intervals of time, and with unisexual animals
inhabiting somewhat separated localities, which can only occasionally come
into contact and pair, the greater vigour and fertility of the crossed
offspring will ultimately prevail in giving uniformity of character to the
individuals of the same species. But when we go beyond the limits of the
same species, free intercrossing is barred by the law of sterility.

In searching for facts which might throw light on the cause of the good
effects from crossing, and of the evil effects from close interbreeding, we
have seen that, on the one hand, it is a widely prevalent and ancient
belief that animals and plants profit from slight changes in their
conditio