292 Darwin, and after Darwin. 



know any of his followers who have made any ap- 

 proach to an adequate use of it in their advocacy 

 of his views. In preparing the present chapter, 

 therefore, I have been particularly careful not to pitch 

 too high my own estimate of its evidential value. 

 That is to say, I have considered, both in the domain 

 of structures and of instincts, what instances admit of 

 being possibly adduced per contra, or as standing out- 

 side the general law that adaptive structures and 

 instincts are of primary use only to their possessors. 

 In the result I can only think of two such instances. 

 These, therefore, I will now dispose of. 



The first was pointed out, and has been fully dis- 

 cussed, by Darwin himself. Certain species of ants 

 are fond of a sweet fluid that is secreted by aphides, 

 and they even keep the aphides as we keep cows for 

 the purpose of profiting by their "milk." Now the 

 point is, that the use of this sweet secretion to the 

 aphis itself has not yet been made out. Of course, if 

 it is of no use to the aphis, it would furnish a case 

 which completely meets Darwin's own challenge. But, 

 even if this supposition did not stand out of analogy 

 with all the other facts of organic nature, most of us 

 would probably deem it prudent to hold that the 

 secretion must primarily be of some use to the aphis 

 itself, although the matter has not been sufficiently 

 investigated to inform us of what this use is. For, in 

 any case, the secretion is not of any vital importance 

 to the ants which feed upon it : and I think but few 

 impartial minds would go so far to save an hypothesis 

 as to maintain, that the Divinity had imposed this drain 

 upon the internal resources of one species of insect 

 for the sole purpose of supplying a luxury to another. 



