occasionally exhibit characters derived from one of their two parents, of
which not a trace could at first be perceived. Certain monstrosities, which
resemble what naturalists call the typical form of the group in question,
apparently come under the same law of reversion. It is assuredly an
astonishing fact that the male and female sexual elements, that buds, and even
full-grown animals, should retain characters, during several generations in
the case of crossed breeds, and during thousands of generations in the case of
pure breeds, written as it were in invisible ink, yet ready at any time to be
evolved under certain conditions.

What these conditions precisely are, we do not know. But any cause which
disturbs the organisation or constitution seems to be sufficient. A cross
certainly gives a strong tendency to the reappearance of long-lost characters,
both corporeal and mental. In the case of plants, this tendency is much
stronger with those species which have been crossed after long cultivation and
which therefore have had their constitutions disturbed by this cause as well
as by crossing, than with species which have always lived under their natural
conditions and have then been crossed. A return, also, of domesticated animals
and cultivated plants to a wild state favours reversion; but the tendency
under these circumstances has been much exaggerated.

When individuals of the same family which differ somewhat, and when races or
species are crossed, the one is often prepotent over the other in transmitting
its character. A race may possess a strong power of inheritance, and yet when
crossed, as we have seen with trumpeter-pigeons, yield to the prepotency of
every other race. Prepotency of transmission may be equal in the two sexes of
the same species, but often runs more strongly in one sex. It plays an
important part in determining the rate at which one race can be modified or
wholly absorbed by repeated crosses with another. We can seldom tell what
makes one race or species prepotent over another; but it sometimes depends on