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. Changes of instinct may sometimes be faci¬ 
litated 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- 
plant, and prevented their attendance during several 
hours. After this interval, I felt sure that the aphides 
