418 The Biology of Flowers 
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’.” 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?, 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 plant®.” We are as 
ignorant of the reason why plants behave differently when crossed 
and self-fertilised as we are in regard to the nature of the differentia- 
tion of the sexual cells, which determines whether a union of the 
sexual cells will prove favourable or unfavourable. 
It is impossible to discuss the different results of cross-fertilisa- 
tion; one point must, however, be emphasised, because Darwin 
attached considerable importance to it. It is inevitable that pollen 
of different kinds must reach the stigma. It was known that pollen 
of the same “species” is dominant over the pollen of another species, 
that, in other words, it is prepotent. Even if the pollen of the same 
1 Cross and Self fertilisation (1st edit.), p. 371. 
2 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 kiinstliche 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. 
3 Cross and Self fertilisation, p. 350. 
