INSTINCT 189 



such gradations ; or we ought at least to be able to 

 show that gradations of some kind are possible ; and 

 this we certainly can do. I have been surprised to 

 find, making allowance for the instincts of animals 

 having been but little observed except in Europe and 

 North America, and for no instinct being known 

 amongst extinct species, how very generally gradations, 

 leading to the most complex instincts, can be discovered. 

 Changes ot instinct may sometimes be facilitated by the 

 same species having different instincts at different 

 periods of life, or at different seasons of the year, or 

 when placed under different circumstances, etc. ; in 

 which case either one or the other instinct might be 

 preserved by natural selection. And such instances of 

 diversity of instinct in the same species can be shown 

 to occur in nature. 



Again as in the case of corporeal structure, and con- 

 formably with my theory, the instinct of each species is 

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

 been produced for the exclusive good of others. 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 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 

 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 antenna? on the abdomen first of one aphis and then 

 of another ; and each aphis, as soon as it felt the 

 antennas, immediately lifted up its abdomen and 

 excreted a limpid drop of sweet juice, which was eagerly 



