Chap. VII. INSTINCT. 211 



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 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 aphis, as soon as it felt the antennae, imme- 

 diately 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. But as the excretion is 

 extremely viscid, it is probably a convenience to the 

 aphides to have it removed ; and therefore probably the 

 aphides do not instinctively excrete for the sole good of 

 the ants. Although I do not believe that any animal in 

 the world performs an action for the exclusive good of 

 another of a distinct species, yet each species tries to 

 take advantage of the instincts of others, as each takes 

 advantage of the weaker bodily structure of others. So 

 again, in some few cases, certain instincts cannot be 

 considered as absolutely perfect ; but as details on this 

 and other such 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 indispensable for the action of natural selection, as 

 many instances as possible ought to have been here 

 given ; but want of space prevents me. I can only 

 assert, that instincts certainly do vary — for instance, 



