| → their 1859 1860 |
| as was first observed by Huber, their 1861 1866 1869 1872 |
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natural selection, except by the slow and gradual accumulation of
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. The canon of "Natura non facit saltum" applies with almost equal force to instincts as to bodily organs. 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
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. |
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as in the case of corporeal structure, and conformably
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
→their
sweet excretion to ants: that they do so voluntarily, the
facts show. I removed all the ants from a group of about a dozen aphides on a
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