THE ORIGIN OF FLOWERS 203 



Here, then, we find an adaptation of certain parts of the moth's 

 body in relation to the fertilization of the flower, but in this ease 

 it is as much in the interest of the moth as of the plant. By- 

 carrying the pollen to the stigma the moths secure the development 

 of the ovules, which serve their offspring as food, so that we have 

 here to do with a peculiar form of care for offspring, which is not 

 more remarkable than many other kinds of brood-care in insects, 

 such as ants, bees, Sphex- wasps, ichneumon-flies, and gall-flies. 



It might be objected that this case of the Yucca is not so much 

 one of eflFecting fertilization as of parasitism ; but the eggs, which are 

 laid in the seed-pods, are very few, and the caterpillars which emerge 

 from them only devour a vei'y small proportion of the seeds, of which 

 there may be about 200 (Fig. 52, B). Thus the plants also derive 

 an advantage from the moth's procedure, for quite enough seeds are 

 left. The form and position of the stamens and of the stigma seem 

 to be as exactly adapted to the visits of the moth as the moth is to 

 the transference of the pollen, for the Yucca can only be fertilized by 

 this one moth, and sets no seed if the moth be absent. For this 

 reason the species of Yucca cultivated in Europe remain sterile. 



Thus the apparent contradiction is explained, and the facts 

 everywhere support the hypothesis that the adaptations between 

 flowers and insects depend upon processes of selection. 



This origin is incontrovertibly proved, it seems to me, in another 

 way, namely, by. the merely relative perfection of the adaptations, or 

 rather, by their relative imperfection. 



I have already pointed out that all adaptations which depend 

 upon natural selection can only be relatively perfect, as follows from 

 the nature of their efficient causes, for natural selection only operates 

 as long as a further increase of the character concerned would be of 

 advantage to the existence of the species. It cannot be operative 

 beyond this point, because the existence of the species cannot be more 

 perfectly secured in this direction, or, to speak more precisely, because 

 further variations in the direction hitherto followed would no longer 

 be improvements, even though they might appear so to us. 



Thus the corolla of many flowers is suited to the thick, hairy 

 head and thorax of the bee, for to these only does the pollen adhere 

 in sufficient quantity to fertilize the next flower ; yet the same flowers 

 are frequently visited by butterflies, and in many of them there has 

 been no adaptation to prevent these useless visits. Obviously this is 

 because preventive arrangements could only begin, according to our 

 theory, when they were necessary to the preservation of the species ; 

 in this case, therefore, only when the pillaging visits of the butterflies 



