60 The Selection Theory 



As in the case of mimicry many species have come to resemble 

 one another through processes of selection, so we know whole classes 

 of phenomena in which plants and animals have become adapted to 

 one another, and have thus been modified to a considerable degree. 

 I refer particularly to the relation between flowers and insects; 

 but as there is an article on " The Biology of Flowers " in this 

 volume, I need not discuss the subject, but will confine myself 

 to pointing out the significance of these remarkable cases for the 

 theory of selection. Darwin has shown that the originally incon- 

 spicuous blossoms of the phanerogams were transformed into flowers 

 through the visits of insects, and that, conversely, several large orders 

 of insects have been gradually modified by their association with 

 flowers, especially as regards the parts of their body actively concerned. 

 Bees and butterflies in particular have become what they are through 

 their relation to flowers. In this case again all that is apparently 

 contradictory to the theory can, on closer investigation, be beautifully 

 interpreted in corroboration of it. Selection can give rise only to 

 what is of use to the organism actually concerned, never to what is 

 of use to some other organism, and we must therefore expect to find 

 that in flowers only characters of use to themselves have arisen, never 

 characters which are of use to insects only, and conversely that in 

 the insects characters useful to them and not merely to the plants 

 would have originated. For a long time it seemed as if an exception 

 to this rule existed in the case of the fertilisation of the yucca 

 blossoms by a little moth, Pronuba yuccasella. This little moth 

 has a sickle-shaped appendage to its mouth-parts which occurs in 

 no other Lepidopteron, and which is used for pushing the yellow 

 pollen into the opening of the pistil, thus fertilising the flower. 

 Thus it appears as if a new structure, which is useful only to the 

 plant, has arisen in the insect. But the difficulty is solved as soon 

 as we learn that the moth lays its eggs in the fruit-buds of the Yucca, 

 and that the larvae, when they emerge, feed on the developing seeds. 

 In effecting the fertilisation of the flower the moth is at the same 

 time making provision for its own offspring, since it is only after 

 fertilisation that the seeds begin to develop. There is thus nothing 

 to prevent our referring this structural adaptation in Pronuba 

 yuccasella to processes of selection, which have gradually trans- 

 formed the maxillary palps of the female into the sickle-shaped 

 instrument for collecting the pollen, and which have at the same 

 time developed in the insect the instinct to press the pollen into 

 the pistil. 



In this domain, then, the theory of selection finds nothing but 

 corroboration, and it would be impossible to substitute for it any 

 other explanation, which, now that the facts are so well known, 



