310 
DARWINISM 
CHAP. 
“Nature abhors perpetual self-fertilisation. 1 In this principle 
we appear to have a sufficient reason for the various con¬ 
trivances by which so many flowers secure cross-fertilisation, 
either constantly or occasionally. These contrivances are so 
numerous, so varied, and often so highly complex and extra¬ 
ordinary, that they have formed the subject of many elaborate 
treatises, and have also been amply popularised in lectures 
and handbooks. It will be unnecessary, therefore, to give 
details here, but the main facts will be summarised in order 
to call attention to some difficulties of the theory which seem 
to require further elucidation. 
Modes of securing Cross-Fertilisation. 
When we examine the various modes in which the cross- 
fertilisation of flowers is brought about, we find that some are 
comparatively simple in their operation and needful adjust¬ 
ments, others highly complex. The simple methods belong to 
four principal classes:—(1) By dichogamy — that is, by the 
anthers and the stigma becoming mature or in a fit state for 
fertilisation at slightly different times on the same plant. The 
result of this is that, as plants in different stations, on different 
soils, or exposed to different aspects flower earlier or later, the 
mature pollen of one plant can only fertilise some plant 
exposed to somewhat different conditions or of different con¬ 
stitution, whose stigma will be mature at the same time ; and 
this difference has been shown by Darwin to be that which is 
adapted to secure the fullest benefit of cross-fertilisation. 
This occurs in Geranium pratense, Thymus serpyllum, Arum 
maculatum, and many others. (2) By the flower being 
self-sterile with its own pollen, as in the crimson flax. This 
absolutely prevents self-fertilisation. (3) By the stamens and 
anthers being so placed that the pollen cannot fall upon the 
stigma, while it does fall upon a visiting insect which carries 
it to the stigma of another flower. This effect is produced in 
a variety of very simple ways, and is often aided by the 
motion of the stamens which bend down out of the way of 
the stigmas before the pollen is ripe, as in Malva sylvestris 
(see Fig. 28). (4) By the male and female flowers being on 
1 For the full detail of his experiments, see Cross- and Self-Fertilisation 
of Plants, 1876. 
