850 THE ORIGIN OF SPECIES 



good for itself, but has never, as far as we can jvidge, 

 been produced for the exclusive good of others. One of 

 the strongest instances of an animal apparently perform- 

 ing 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 I could, as the ants do with 

 their antennse; but not one excreted. Afterward I al- 

 lowed 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 ob- 

 servations 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 excretion 

 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 





