THE ORIGIN OF FLOWERS 189 



to the nectar, and at the same time to the pollen and the stigma. 

 And then there is the diversity in the form and colour of the ' honey- 

 guides ' on the ' alighting surface,' that is, the under lip of the flower, 

 upon which the insect sits and holds fast, while it pushes its head as 

 far as possible into the spur, so that its proboscis may reach the nectar 

 lying deep within it ! Even though we cannot pretend to guess at the 

 significance of every curve and colour-spot in one of the great tropical 

 orchids, such as Stanhopea tigrina, yet we may believe, with Sprengel, 

 that all this has its significance, or has had it for the ancestors of the 

 plant in question, and in fact that the flower is made up of nothing 

 but adaptations, either actual or inherited from its ancestors, although 

 sometimes perhaps no longer of functional importance. 



So far, then, we have illustrated the fact that there are hundreds 

 and thousands of contrivances in flowers adapted solely to the visits 

 of insects and to securing cross -fertilization, and these adaptations go 

 so far that we might almost believe them to be the outcome of the 

 most exact calculation and the most ingenious reflection. But they all 

 admit of interpretation through natural selection, for all these details, 

 which used to be looked upon as merely ornamental, are directly or 

 indirectly of use to the species; directly, when, for instance, they concern 

 the dusting of the insect with the pollen ; indirectly, when they are 

 a means of attracting visits. 



Moreover, the evidence of the operation of the processes of selec- 

 tion becomes absolutely convincing when we consider that, as in 

 symbiosis, there are always two sets of adaptations taking place 

 independently of one another — those of the flowers to the visits of 

 the insects, and those of the insects to the habit of visiting the flowers. 

 To understand this clearly we must turn our attention to the insects, 

 and try to see in what way they have been changed by adapting 

 themselves to the diet which the flowers afford. 



As is well known, several orders of insects possess mouth-parts 

 which are suited for sucking up fluids, and these have evolved, through 

 adaptation to a fluid diet, from the biting mouth-parts of the primitive 

 insects which we see still surviving in several orders. Thus the 

 Diptera may have gradually acquired the sucking proboscis which 

 occurs in many of them by licking up decaying vegetable and animal 

 matter, and by piercing into and sucking living animals. But even 

 among the Diptera several families have more recently adapted them- 

 selves quite specially to a flower diet, to honey-sucking, like the 

 hover-flies, the Syrphidse, and the Bombyliidae, whose long thin proboscis 

 penetrates deep into narrow corolla-tubes, and is able to suck up the 

 nectar from the very bottom The transformation was not so impor- 



I. N 



