It has already been shown that there is no close relation between the number of seeds produced by flowers when crossed and self-fertilised, and the degree to which their offspring are aaffected by the two processes.  I have also given reasons for believing that the inefficiency of a plant’s own pollen is in most cases an incidental result, or has not been specially acquired for the sake of preventing self-fertilisation.  On the other hand, there can hardly be a doubt that dichogamy, which prevails according to Hildebrand in the greater number of species (10/23.  ‘Die Geschlecter Vertheiling’ etc. page 32.),—­that the heterostyled condition of certain plants,—­and that many mechanical structures—­have all been acquired so as both to check self-fertilisation and to favour cross-fertilisation.  The means for favouring cross-fertilisation must have been acquired before those which prevent self-fertilisation; as it would manifestly be injurious to a plant that its stigma should fail to receive its own pollen, unless it had already become well adapted for receiving pollen from another individual.  It should be observed that many plants still possess a high power of self-fertilisation, although their flowers are excellently constructed for cross-fertilisation—­for instance, those of many papilionaceous species.

It may be admitted as almost certain that some structures, such as a narrow elongated nectary, or a long tubular corolla, have been developed in order that certain kinds of insects alone should obtain the nectar.  These insects would thus find a store of nectar preserved from the attacks of other insects; and they would thus be led to visit frequently such flowers and to carry pollen from one to the other. (10/24.  See the interesting discussion on this subject by Hermann Muller, ’Die Befruchtung’ etc. page 431.) It might perhaps have been expected that plants having their flowers thus peculiarly constructed would profit in a greater degree by being crossed, than ordinary or simple flowers; but this does not seem to hold good.  Thus Tropaeolum minus has a long nectary and an irregular corolla, whilst Limnanthes douglasii has a regular flower and no proper nectary, yet the crossed seedlings of both species are to the self-fertilised in height as 100 to 79.  Salvia coccinea has an irregular corolla, with a curious apparatus by which insects depress the stamens, while the flowers of Ipomoea are regular; and the crossed seedlings of the former are in height to the self-fertilised as 100 to 76, whilst those of the Ipomoea are as 100 to 77.  Fagopyrum is dimorphic, and Anagallis collina is non-dimorphic, and the crossed seedlings of both are in height to the self-fertilised as 100 to 69.