fertility--measured by the number or weight of the seeds produced
by an equal number of plants--noticed under different conditions of
fertilisation may be quoted in illustration.

On crossing On crossing On self-
with a fresh plants of the fertilisation
stock same stock
Mimuleus luteus
(First and ninth generation) 100 4 3

Eschscholzia californica
(second generation) 100 45 40

Dianthus caryophyllus
(third and fourth generation) 100 45 33

Petunia violacea 100 54 46

Crossing under very similar conditions shows, therefore, that the
difference between the sexual cells is smaller and thus the result of
crossing is only slightly superior to that given by self-fertilisation.
Is, then, the favourable result of crossing with a foreign stock to be
attributed to the fact that this belongs to another systematic entity
or to the fact that the plants, though belonging to the same entity
were exposed to different conditions? This is a point on which further
researches must be taken into account, especially since the analysis of
the systematic entities has been much more thorough than formerly. (In
the case of garden plants, as Darwin to a large extent claimed, it
is not easy to say whether two individuals really belong to the same
variety, as they are usually of hybrid origin. In some instances
(Petunia, Iberis) the fresh stock employed by Darwin possessed flowers
differing in colour from those of the plant crossed with it.) We
know that most of Linneaus's species are compound species, frequently
consisting of a very large number of smaller or elementary species
formerly included under the comprehensive term varieties. Hybridisation
has in most cases affected our garden and cultivated plants so that they
do not represent pure species but a mixture of species.

But this consideration has no essential bearing on Darwin's point of
view, according to which the nature of the sexual cells is influenced 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 generall