3l6 SIGNIFICANCE OF SEXUAI REPRODUCTION [V. 



kind — gradually developed into corollas with the honey deeply 

 concealed. The whole process was presumably first started 

 by the flower, for the gradual withdrawal of the honey to 

 greater depths conferred the advantage of protection from rain 

 (Hermann Miiller), while larger quantities of honey could be 

 stored up, and this would also increase the number of insects 

 visiting the flower and render their visits more certain. As 

 soon as this withdrawal occurred, the mouth-parts of insects 

 began to be subjected to a selective process whereby these 

 organs in some of them were lengthened at the same rate as 

 that at which the honey was withdrawn. When once the 

 process had begun, its continuance was ensured, for as soon 

 as flower-frequenting insects were divided into two groups 

 with short and with long mouth-parts respectively, a further 

 increase in the length of the corolla-tube necessarily took place 

 in all those flowers which were especially benefited by the 

 assured visits of a relatively small number of species of insects, 

 viz., those flowers in which cross-fertilization was more cer- 

 tainly 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 certainly per- 

 formed 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 com- 

 pletety 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 diflerent 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 but- 

 terfly which brings about fertilization, would have continued to 

 increase in length as long as it remained advantageous for the 

 flower to exclude 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 



