210 INSTINCT. Chap. VII. 



natural selection, except by the slow and gradual accu- 

 mulation 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 gra- 

 dations, leading to the most complex instincts, can be 

 discovered. The canon of "Natura non facit saltum" 

 applies with almost equal force to instincts as to bodily 

 organs. Changes of instinct may sometimes be facili- 

 tated 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 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 per- 

 forming 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- 



