INSTINCT. 245 



for itself; but has never, as far as we can judge, been pro- 

 duced for the exclusive good of others. One of the strong- 

 est instances 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 re- 

 moved 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 I could, as the ants do with their antennse; but not 

 one excreted. Afterward, 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 begun to play with its antennae on the abdomen 

 first of one aphis and then of another; and each, as soon as 

 it felt the antennge, immediately lifted up its abdomen and 

 excreted a limped drop of sweet juice, which was eagerly 

 devoured by the ant. Even the quite young aphides be- 

 haved in this manner, showing that the action was instinc- 

 tive, and not the result of experience. It is certain, from 

 the observations of Huber, that the aphides show no dis- 

 like to the ants: if the latter be not present they are at 

 last compelled to eject their excretion. But as the excre- 

 tion 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. Althougli 

 there is no evidence that any animal performs an action 

 for the exclusive good of another species, yet each tries to 

 take advantage of the instincts of others, as each takes 

 advantage of the weaker bodily structure of other 

 species. So again certain instincts can not be considered as 

 absolutely perfect; but as details on this and other sucli 

 points are not indispensable, 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 as possible ought to be given; but want of space 



