Cross-fertilisation 417 
fluenced by external conditions. Even individuals growing close to 
one another are only apparently exposed to identical conditions. 
Their sexual cells may therefore be differently influenced and thus 
give favourable results on crossing, as “the benefits which so 
generally 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 con- 
ditions. 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 genera- 
tion 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' has recently shown that species of Ribes cultivated 
under unnatural conditions frequently produce a mixed (ie. 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 sub- 
jected, is securely fortified on all sides®.” 
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’, crossing between organisms which 
have been exposed to different conditions becomes still more ad- 
vantageous. 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 
sexual cells of plants which have lived under the same or only 
slightly different conditions. All the wonderful arrangements for 
1 Janezewski, ‘‘Sur les anthéres stériles des Groseilliers,” Bull. de Vacad. des sciences 
de Cracovie, June, 1908. 
? Cross and Self fertilisation (1st edit.), p, 444. 
> Reasons for this are given by Darwin in Variation under Domestication (2nd edit.), 
Vol. 1. p. 127. 
D. 27 
