Chap. VII. 
INSTINCT. 
211 
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 be here given ; but 
want of space prevents me. I can only assert, that 
instincts certainly do vary^—for instance, the migratory 
instinct, both in extent and direction, and in its total 
loss. So it is with the nests of birds, which vary partly 
