y follow from a cross
between two plants apparently depend on the two differing somewhat in
constitution or character." As a matter of fact we are familiar with a
large number of cases in which the condition of the reproductive organs
is influenced by external conditions. Darwin has himself demonstrated
this for self-sterile plants, that is plants in which self-fertilisation
produces no result. This self-sterility is affected by climatic
conditions: thus in Brazil Eschscholzia californica is absolutely
sterile to the pollen of its own flowers; the descendants of Brazilian
plants in Darwin's cultures were partially self-fertile in one
generation and in a second generation still more so. If one has any
doubt in this case whether it is a question of the condition of
the style and stigma, which possibly prevents the entrance of the
pollen-tube or even its development, rather than that of the actual
sexual cells, in other cases there is no doubt that an influence is
exerted on the latter.

Janczewski (Janczewski, "Sur les antheres steriles des Groseilliers",
"Bull. de l'acad. des sciences de Cracovie", June, 1908.) has recently
shown that species of Ribes cultivated under unnatural conditions
frequently produce a mixed (i.e. partly useless) or completely sterile
pollen, precisely as happens with hybrids. There are, therefore,
substantial reasons for the conclusion that conditions of life exert an
influence on the sexual cells. "Thus the proposition that the benefit
from cross-fertilisation depends on the plants which are crossed
having been subjected during previous generations to somewhat different
conditions, or to their having varied from some unknown cause as if they
had been thus subjected, is securely fortified on all sides." ("Cross
and Self fertilisation" (1st edition), page 444.)

We thus obtain an insight into the significance of sexuality. If an
occasional and slight alteration in the conditions under which plants
and animals live is beneficial (Reasons for this are given by Darwin
in "Variation under Domestication" (2nd edition), Vol. II. page
127.), crossing between organisms which have been exposed to different
conditions becomes still more advantageous. The entire constitution
is in this way influenced from the beginning, at a time when the whole
organisation is in a highly plastic state. The total life-energy, so
to speak, is increased, a gain which is not produced by asexual
reproduction or by the union of