sexual cells of plants which have lived
under the same or only slightly different conditions. All the
wonderful arrangements for cross-fertilisation now appear to be useful
adaptations. Darwin was, however, far from giving undue prominence to
this point of view, though this has been to some extent done by others.
He particularly emphasised the following consideration:--"But we should
always keep in mind that two somewhat opposed ends have to be gained;
the first and more important one being the production of seeds by
any means, and the second, cross-fertilisation." ("Cross and Self
fertilisation" (1st edition), page 371.) Just as in some orchids and
cleistogamic flowers self-pollination regularly occurs, so it may also
occur in other cases. Darwin showed that Pisum sativum and Lathyrus
odoratus belong to plants in which self-pollination is regularly
effected, and that this accounts for the constancy of certain sorts of
these plants, while a variety of form is produced by crossing. Indeed
among his culture plants were some which derived no benefit from
crossing. Thus in the sixth self-fertilised generation of his Ipomoea
cultures the "Hero" made its appearance, a form slightly exceeding its
crossed companion in height; this was in the highest degree self-fertile
and handed on its characteristics to both children and grandchildren.
Similar forms were found in Mimulus luteus and Nicotiana (In Pisum
sativum also the crossing of two individuals of the same variety
produced no advantage; Darwin attributed this to the fact that the
plants had for several generations been self-fertilised and in each
generation cultivated under almost the same conditions. Tschermak
("Ueber kunstliche Kreuzung an Pisum sativum") afterwards recorded the
same result; but he found on crossing different varieties that usually
there was no superiority as regards height over the products of
self-fertilisation, while Darwin found a greater height represented by
the ratios 100:75 and 100:60.), types which, after self-fertilisation,
have an enhanced power of seed-production and of attaining a greater
height than the plants of the corresponding generation which are crossed
together and self-fertilised and grown under the same conditions.
"Some observations made on other plants lead me to suspect that
self-fertilisation is in some respects beneficial; although the benefit
thus derived is as a rule very small compared with that from a cross
with a distinct p