30& THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



fertilization was more certainly performed in this way than by the 

 uncertain visits of a great variety of species. This would imply 

 that a still further increase in length would take place, for it is 

 obvious that the cross-fertilization of any flower would be more cer- 

 tainly performed by an insect when the number of species of plants 

 visited by it became less ; and hence the cross- fertilization would 

 be rendered most certain when the insect became completely 

 adapted in size, form, character of its surface, and the manner in 

 which it obtained the honey to the peculiarities of the flower. 

 Those insects which obtain honey from a great variety of flowers 

 are sure to waste a great part of the pollen by carrying it to the 

 flowers of many different species, while insects which can only 

 obtain honey from a few species of plants must necessarily visit 

 many flowers of the same species one after the other, and they 

 would therefore more generally distribute the pollen in an effective 

 manner. 



Hence the tube of the corolla, and the ' tongue ' of the butterfly 

 which brings about fertilization, would have continued to increase 

 in length as long as it remained advantageous for the flower to ex- 

 clude other less useful visitors, and as long as it was advantageous 

 for the butterfly to secure the sole possession of the flower. Hence 

 there is no competition between the flower and the butterfly which 

 fertilizes it, but between these two on the one side, and the other 

 would-be visitors of the flower on the other. Further details as to 

 the advantages which the flower gains by excluding all other 

 visitors, and the butterfly by being the only visitor of the flower, 

 and also as to the manifold and elaborate mutual adaptations between 

 insects and flowers, and as to the advantages and disadvantages 

 which follow from the concealment of the honey will be found in 

 Hermann Miiller's J work on the fertilization of flowers, in which 

 all these subjects are minutely discussed, and are clearly explained 

 in a most admirable manner. 



APPENDIX III. ADAPTATIONS IN PLANTS 2 . 



It is well known that Christian Conrad Sprengel was the first 

 to recognise that the forms and colours of flowers are not due to 



1 English Edition, translated by D'Arcy W. Thompson, B.A. London, 1883, 

 p. 509 et aeqq. 



8 Appendix to page 260. 



