252 PHYSIOLOGICAL "DEVELOPMENT. 



dered at the same time more varied by the special require- 

 ments of the insects and of the plants in each locality, under 

 each change of conditions. Of course, the 



genesis of the sweet secretions and the odours of flowers, 

 has a parallel interpretation. The simultaneous production 

 of honey, or some kindred substance, is implied above ; 

 since, unless a bait co- existed with the colour, the colour 

 would not attract insects, and would not be maintained 

 and intensified by natural selection. Gums, and resins, 

 and balsams, are familiar products of plants ; apparently, 

 in many cases, excreted as useless matters from various 

 parts of their surfaces. These substances, admitting of 

 wide variations in quality, as they do, afford opportunities 

 for the action of natural selection wdierever any of them 

 attractive to insects, happen to be produced near the organs 

 of fructification. And this action of natural selection once 

 set up, may lead to the establishment of a local excretion, to 

 the production of an excretion more and more attractive, and 

 to the disposal of the organ containing it in such a way as 

 most to facilitate the carrying away of pollen. Similarly 

 and simultaneously with odours. Odours, like colours, draw 

 insects to flowers. After observing how Bees come swarming 

 into a house where honey is largely exposed, or how Wasps 

 find their way into a shop containing much ripe fruit, it 

 cannot be questioned that insects are to a considerable extent 

 guided by scent. Being thus sensitive to the aromatic sub- 

 stances which flowers exhale, they may, when the flowers are 

 in large masses, be attracted by them from distances at which 

 the flowers themselves are invisible. And manifestly, the 

 flowers which so attract them from the greatest distances, 

 increasing thereby their chances of efficient fertilization, will 

 be most likely to perpetuate themselves. That is to say, 

 survival of the fittest must tend to produce perfumes that 

 are both more powerful and more attractive. 



These physiological differentiations, then, which mark off 

 the foliar organs of flowers from other foliar organs, are 



