One of the strongest 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 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.