INSTINCT 265 



Again, as in the case of corporeal structure, and conform- 

 ably to my theory, the instinct of each species is good for 

 itself, but has never, as far as we can judge, been produced 

 for the exclusive good of others. One of the strongest in- 

 stances of an animal apparently performing an action for the 

 sole good of another, with which I am acquainted, is that of 

 aphides voluntarily yielding, as was first observed by Huber, 

 their sweet excretion to ants : that they do so voluntarily, the 

 following facts show. I removed all the ants from a group 

 of about a dozen aphides on a dock-plant, and prevented their 

 attendance during several hours. After this interval, I felt 

 sure that the aphides would want to excrete. I watched them 

 for some time through a lens, but not one excreted; I then 

 tickled and stroked them with a hair in the same manner, as 

 well as T could, as the ants do with their antennae ; but not one 

 excreted. Afterwards I allowed an ant to visit them, and it 

 immediately seemed, by its eager way of running about, to be 

 well aware what a rich flock it had discovered ; it then began 

 to play with its antennae on the abdomen first of one aphis and 

 then of another ; and each, as soon as it felt the antennae, 

 immediately lifted up its abdomen and excreted a limpid drop 

 of sweet juice, which was eagerly devoured by the ant. Even 

 the quite young aphides behaved in this manner, showing that 

 the action was instinctive, and not the result of experience. 

 It is certain, from the observations of Huber, that the aphides 

 show no dislike to the ants : if the latter be not present they 

 are at last compelled to eject their excretion. But as the ex- 

 cretion is extremely viscid, it is no doubt a convenience to 

 the aphides to have it removed ; therefore probably they do 

 not excrete solely for the good of the ants. Although there 

 is no evidence that any animal performs an action for the 

 exclusive good of another species, yet each tries to take ad- 

 vantage of the instincts of others, as each takes advantage 

 of the weaker bodily structure of other species. So again 

 certain instincts cannot be considered as absolutely perfect; 

 but as details on this and other such points are not indis- 

 pensable, they may be here passed over. 



As some degree of variation in instincts under a state of 

 nature, and the inheritance of such variations, are indis- 

 pensable for the action of natural selection, as many instances 



