ility to the
offspring, than does the same hybrid when used as the mother. Lastly,
it is certain that variability may be transmitted through either sexual
element, whether or not originally excited in them, for Koelreuter and
Gaertner[655] found that when two species were crossed, if either one
was variable, the offspring were rendered variable.

* * * * *

_Summary._--From the facts given in this chapter, we may conclude that the
variability of organic beings under domestication, although so general, is
not an inevitable contingent on growth and reproduction, but results from
the conditions to which the parents have been exposed. Changes of any kind
in the conditions of life, even extremely slight changes, often suffice to
cause variability. Excess of nutriment is perhaps the most efficient single
exciting cause. Animals and plants continue to be variable for an immense
period after their first domestication; but the conditions to which they
are exposed never long remain quite constant. In the course of time they
can be habituated to certain changes, so as to become less variable; and it
is possible that when first domesticated they may have been even more
variable than at present. There is good evidence that the power of changed
conditions accumulates; so that two, three, or more generations must be
exposed to new conditions before any effect is visible. The crossing of
distinct forms, which have already become variable, increases in the
offspring the tendency to further variability, by the unequal commingling
of the characters of the two parents, by the reappearance of long-lost
characters, and by the appearance of absolutely new characters. Some
variations are induced by the direct action of the surrounding conditions
on the whole organisation, or on certain parts alone, and other variations
are induced indirectly through the reproductive system being affected in
the same manner as is so common with organic beings when removed from their
natural conditions. The causes which induce variability act on the mature
organism, on the embryo, and, as we have good reason to believe, on both
sexual elements before impregnation has been effected.

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CHAPTER XXIII.

DIRECT AND DEFINITE ACTION OF THE EXTERNAL CONDITIONS OF LIFE.

SLIGHT MODIFICATIONS IN PLANTS FROM THE DEFINITE ACTION OF CHANGED
CONDITIONS IN SIZE,