equently
they display an almost infinite diversity of character. He who wishes,
says Koelreuter,[643] to obtain an endless number of varieties from
hybrids should cross and recross them. There is also much variability
when hybrids or mongrels are reduced or absorbed by repeated crosses
with either pure parent-form; and a still higher degree of variability
when three distinct species, and most of all when four species, are
blended together by successive crosses. Beyond this point
Gaertner,[644] on whose authority the foregoing statements are made,
never succeeded in effecting a union; but Max Wichura[645] united six
distinct species of willows into a single hybrid. The sex of the
parent-species affects in an inexplicable manner the degree of
variability of hybrids; for Gaertner[646] repeatedly found that when a
hybrid was used as the father, and either one of the pure
parent-species, or a third species, was used as the mother, the
offspring were more variable than when the same hybrid was used as the
mother, and either pure parent or the same third species as the father:
thus seedlings from _Dianthus barbatus_ crossed by the hybrid _D.
chinensi-barbatus_ were more variable than those raised from this
latter hybrid fertilised by the pure _D. barbatus_. Max Wichura[647]
insists strongly on an analogous result with his hybrid willows. Again
Gaertner[648] asserts that the degree of variability sometimes differs
in hybrids raised from reciprocal crosses between the same two species;
and here the sole difference is, that the one species is first used as
the father and then as the mother. On the whole we see that,
independently of the appearance of new characters, the variability of
successive crossed generations is extremely complex, partly from the
offspring partaking unequally of the characters of the two
parent-forms, and more especially from their unequal tendency to revert
to these same characters or to those of more ancient progenitors.

* * * * *

_On the Manner and on the Period of Action of the Causes which induce
Variability._--This is an extremely obscure subject, and we need here only
briefly consider, firstly, whether inherited variations are caused by the
organisation being directly acted on, or indirectly through the
reproductive system; and secondly, at what period o