which has been contaminated by a single cross, nor when, in the attempt to
form an intermediate breed, half-bred animals have been matched together
during many generations, can any rule be laid down how soon the tendency to
reversion will be obliterated. It depends on the difference in the strength or
prepotency of transmission in the two parent-forms, on their actual amount of
difference, and on the nature of the conditions of life to which the crossed
offspring are exposed. But we must be careful not to confound these cases of
reversion to characters which were gained by a cross, with those under the
first class, in which characters originally common to BOTH parents, but lost
at some former period, reappear; for such characters may recur after an almost
indefinite number of generations.

The law of reversion is as powerful with hybrids, when they are sufficiently
fertile to breed together, or when they are repeatedly crossed with either
pure parent-form, as in the case of mongrels. It is not necessary to give
instances. With plants almost every one who has worked on this subject, from
the time of Kolreuter to the present day, has insisted on this tendency.
Gartner has recorded some good instances; but no one has given more striking
ones than Naudin. (13/19. Kolreuter gives curious cases in his 'Dritte
Fortsetzung' 1766 ss. 53, 59; and in his well-known 'Memoirs on Lavatera and
Jalapa.' Gartner 'Bastarderzeugung' ss. 437, 441, etc. Naudin in his
"Recherches sur l'Hybridite" 'Nouvelles Archives du Museum' tome 1 page 25.)
The tendency differs in degree or strength in different groups, and partly
depends, as we shall presently see, on whether the parent-plants have been
long cultivated. Although the tendency to reversion is extremely general with
nearly all mongrels and hybrids, it cannot be considered as invariably
characteristic of them; it may also be mastered by long-continued selection;
but these subjects will more properly be discussed in a future chapter on
Crossing. From what we see of the power and scope of reversion, both in pure
races, and when varieties or species are crossed, we may infer that characters