XXII 



of cases, a flower is a piece of mechanism the object of which is to 

 convert insect visitors into agents of fertilisation. Sprengel's obser- 

 vations had been most undeservedly neglected and well-nigh for- 

 gotten ; but Robert Brown having directed Darwin's attention to 

 them in 1841, he was attracted towards the subject, and verified many 

 of Sprengel's statements. (Ill, p. 258.) It may be doubted whether 

 there was a living botanical specialist, except perhaps Brown, who 

 had done as much. If, however, adaptations of this kind were to be 

 explained by natural selection, it was necessary to show that the 

 plants which were provided with mechanisms for ensuring the aid 

 of insects as fertilisers, were by so much the better fitted to 

 compete with their rivals. This Sprengel had not done. Darwin 

 had been attending to cross fertilisation in plants, so far back as 

 1839, from having arrived in the course of his speculations on the 

 origin of species "that crossing played an important part in keeping 

 specific forms constant" (I, p. 90). The further development of his 

 views on the importance of cross fertilisation appears to have taken 

 place between this time and 1857, when he published his first papers 

 on the fertilisation of flowers in the ' Gardener's Chronicle.' If the 

 conclusion at which he ultimately arrived, that cross fertilisation is 

 favourable to the fertility of the parent and to the vigour of the off- 

 spring, is correct, then it follows that all those mechanisms which 

 hinder self-fertilisation and favour crossing must be advantageous in 

 the struggle for existence ; and, the more perfect the action of the 

 mechanism, the greater the advantag-e. Thus the way lay open for the 

 operation of natural selection in gradually perfecting the flower as a 

 fertilisation-trap. Analogous reasoning applies to the fertilising insect. 

 The better its structure is adapted to that of the trap, the more will 

 it be able to profit by the bait, whether of honey or of pollen, to the 

 exclusion of its competitors. Thus, by a sort of action and reaction, 

 a two-fold series of adaptive modifications will be brought about. 



In 1865, the important bearing of this subject on his theory led 

 Darwin to commence a great series of laborious and difficult expe- 

 riments on the fertilisation of plants, which occupied him for 

 eleven years, and furnished him with the unexpectedly strong evi- 

 dence in favour of the influence of crossing which he published in 

 1876, under the title of ' The Effects of Cross and Self Fertilisation in 

 the Vegetable Kingdom.' Incidentally, as it were, to this heavy piece 

 of work, he made the remarkable series of observations on the 

 different arrangements by which crossing is favoured and, in many 

 cases, necessitated, which appeared in the work on ' The Different 

 Forms of Flowers in Plants of the same Species ' in 1877. 



In the course of the twenty years during which Darwin was thus 

 occupied in opening up new regions of investigation to the botanist 

 tind showing the profound physiological significance of the apparently 



