Chap. VIII. 



Instinct. 



207 



of what may be called spontaneous variations of instincts ;— that 

 is of variations produced by the same unknown causes which pro- 

 duce slight deviations of bodily structure. 



No complex instinct can possibly be produced through natural 

 selection, except by the slow and gradual accumulation of numerous 

 slight, yet profitable, variations. Hence, as in the case of corporeal 

 structures, we ought to find in nature, not the actual transitional 

 gradations by which each complex instinct has been acquired— for 

 these could be found only in the lineal ancestors of each species — 

 but we ought to find in the collateral lines of descent some evidence 

 of 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 of 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, &c. ; in which case either the 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 conformably to 

 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. On e of the strongest instances of an animal appare ntly, 

 .performing an action for the sole good of another, with w hich I anT 

 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. After- 

 wards 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 antennse on 

 the abdomen first of one aphis and then of another; and each, as 

 soon as it felt the antennas, immediately lifted up its abdomen and 

 excreted a limpid drop of sweet juice, which was eagerly devoured 



