16G THE EVOLUTION THEORY 



slu'll unless its instinct compelled it thereto, just as its instinct impels 

 it to cover itself with sea-anemones, and fearlessly to grasp the 

 dani^orous animal, -which, however, only shows its partner its softer 

 side. Truly, such transformations of instinct are wonderful enough, 

 hut that they should have come about through intelligence is here 

 (piite inconceivable: there remains nothing but natural selection. 



A case in which no apparent corporeal adaptations have occurred, 

 but which depends altogether on slight modifications of the instincts, 

 is atibrded by the well-known relations between ants and aphides. 

 These two groups of insects live in a kind of symbiosis, although 

 they are by no means inseparably connected with each other. 

 AVherever strong colonies of aphides cover the young shoots of 

 a plant, such as a stinging-nettle, a rose, or an elder, we almost always 

 find ants which walk cautiously about among the plant-lice, often in 

 great numbers, stopping now and again to stroke them with their 

 antennffi, and then licking up the sweet juice from the intestine which 

 they now give forth. Darwin showed by experiment that the 

 aphides retain this juice if no ants are on the spot, and only give 

 it off when ants are put beside them. Herein lies the proof that w^e 

 have again to do with a case of modification of instincts. This 

 juice is, of course, not the secretion of special glands, as it was still 

 believed to be in Darwin's time, and it does not come from the 

 so-called ' honey-tubes ' situated on the back of the abdomen of the 

 aphides ; it is simply their excrement, wdiich is liquid like their food, 

 and the voiding of it has become instinctively connected with the 

 presence of the friendly ants. 



That the aphides are not in any way afraid of the ants implies, 

 in itself, a modification of their instinct, for these poisonous insects, 

 prone to biting, are otherwise much dreaded in the insect w^orld. 

 Moreover, the aphides, harmless as they seem, are not quite without 

 means of defence, although these are never used ag-ainst the ants, 

 other animals which approach them they bespatter with the sticky, 

 oily secretion prepared in the so-called ' honey-tubes ' already noted, 

 squirting it especially into the eyes of an assailant, so that the attack 

 is abandoned. 



Of course the aphides have no idea wherein the utility of their 

 friendship for the ants consists, but it is not difficult for us to 

 discover it, since the ants, by their mere presence in the aphid 

 colony, frighten and keep oft' their enemies. We see, then, that 

 the conditions for a process of natural selection are here afforded : the 

 instinct to be friendly to the ants is thoroughlj^ useful, and the 

 instinct of the ants to seek out the aphides, and, instead of devouring 



