As to natural varieties or races under normal conditions, sexually
propagated, it could readily be shown that they are neither more nor less
likely to disappear from any inherent cause than the species from which
they originated. Whether species wear out, i.e., have their rise,
culmination, and decline, from any inherent cause, is wholly a geological
and very speculative problem, upon which, indeed, only vague conjectures can
be offered. The matter actually under discussion concerns cultivated
domesticated varieties only, and, as to plants, is covered by two
questions.

First, Will races propagated by seed, being so fixed that they come true to
seed, and purely bred (not crossed with any other sort), continue so
indefinitely, or will they run out in time--not die out, perhaps, but lose
their distinguishing characters? Upon this, all we are able to say is that
we know no reason why they should wear out or deteriorate from any inherent
cause. The transient existence or the deterioration and disappearance of
many such races are sufficiently accounted for otherwise; as in the case of
extraordinarily exuberant varieties, such as mammoth fruits or roots, by
increased liability to disease, already adverted to, or by the failure of
the high feeding they demand. A common cause, in ordinary cases, is
cross-breeding, through the agency of wind or insects, which is difficult
to guard against. Or they go out of fashion and are superseded by others
thought to be better, and so the old ones disappear.

Or, finally, they may revert to an ancestral form. As offspring tend to
resemble grandparents almost as much as parents, and as a line of
close-bred ancestry is generally prepotent, so newly-originated varieties
have always a tendency to reversion. This is pretty sure to show itself in
some of the progeny of the earlier generations, and the breeder has to
guard against it by rigid selection. But the older the variety is--that is,