orm.[927] All these cases and degrees of reversion
incessantly occur.

It was shown in the fifteenth chapter that certain characters are
antagonistic to each other or do not readily blend together; hence, when
two animals with antagonistic characters are crossed, it might well happen
that a sufficiency of gemmules in the male alone for the reproduction of
his peculiar characters, and in the female alone for the reproduction of
her peculiar characters, would not be present; and in this case dormant
gemmules derived from some remote progenitor might easily gain the
ascendency, and cause the reappearance of long-lost characters. For
instance, when black and white pigeons, or black and white fowls, are
crossed,--colours which do not readily blend,--blue plumage in the one
case, evidently derived from the rock-pigeon, and red plumage in the other
case, derived from the wild jungle-cock, occasionally reappear. With
uncrossed breeds the same result would follow, under conditions which
favoured the multiplication and development of certain dormant gemmules, as
when animals become feral and revert to their pristine character. A certain
number of gemmules being requisite for the development of each character,
as is known to be the case from several spermatozoa or pollen-grains being
necessary for fertilisation, and time favouring their multiplication, will
together account for the curious cases, insisted on by Mr. Sedgwick, of
certain diseases regularly appearing in alternate generations. This
likewise holds good, more or less strictly, with other weakly inherited
modifications. Hence, as I have heard it remarked, certain diseases appear
actually to gain strength by the intermission of a generation. The
transmission of dormant gemmules during many successive generations is
hardly in itself more improbable, as {402} previously remarked, than the
retention during many ages of rudimentary organs, or even only of a
tendency to the production of a rudiment; but there is no reason to suppose
that all dormant gemmules would be transmitted and propagated for ever.
Excessively minute and numerous as they are believed to be, an infinite
number derived, during a long course of modification and descent, from each
cell of each progenitor, could not be supported or nourished by the
organism. On the other hand, it does not seem improbable that certain
gemmules, under favourable conditions, should be retained and go on
multiplying for a longer pe