s not applicable to the
reappearance of characters lost long ago by variation; and it is hardly
applicable to races or species which, after having been crossed at some
former period with a distinct form, and having since lost all traces of the
cross, nevertheless occasionally yield an individual which reverts (as in
the case of the great-great-grandchild of the pointer Sappho) to the
crossing form. The most simple case of reversion, namely, of a hybrid or
mongrel to its grandparents, is connected by an almost perfect series with
the extreme case of a purely-bred race recovering characters which had been
lost during many ages; and we are thus led to infer that all the cases must
be related by some common bond.

Gaertner believed that only those hybrid plants which are highly sterile
exhibit any tendency to reversion to their parent-forms. It is rash to
doubt so good an observer, but this conclusion must I think be an error;
and it may perhaps be accounted for by the nature of the genera observed by
him, for he admits that the tendency differs in different genera. The
statement is also directly contradicted by Naudin's observations, and by
the notorious fact that perfectly fertile mongrels exhibit the tendency in
a high degree,--even in a higher degree, according to Gaertner himself,
than hybrids.[114]

Gaertner further states that reversions rarely occur with {50} hybrid
plants raised from species which have not been cultivated, whilst, with
those which have been long cultivated, they are of frequent occurrence.
This conclusion explains a curious discrepancy: Max Wichura,[115] who
worked exclusively on willows, which had not been subjected to culture,
never saw an instance of reversion; and he goes so far as to suspect that
the careful Gaertner had not sufficiently protected his hybrids from the
pollen of the parent-species: Naudin, on the other hand, who chiefly
experimented on cucurbitaceous and other cultivated plants, insists more
strenuously than any other author on the tendency to reversion in all
hybrids. The conclusion that the condition of the parent-species, as
affected by culture, is one of the proximate causes leading to reversion,
agrees fairly well with the converse case of domesticated animals and
cultivated plants being liable to reversion when they become feral; for in
both cases the organisation or constitution must be disturbed, though in a
very different way.

Finally, we have seen that characters o