With respect to the means by which this strange
and odious instinct was acquired, if it were of great importance for the
young cuckoo, as is probably the case, to receive as much food as possible
soon after birth, I can see no special difficulty in its having gradually
acquired, during successive generations, the blind desire, the strength,
and structure necessary for the work of ejection; for those cuckoos which
had such habits and structure best developed would be the most securely
reared. The first step towards the acquisition of the proper instinct
might have been mere unintentional restlessness on the part of the young
bird, when somewhat advanced in age and strength; the habit having been
afterwards improved, and transmitted to an earlier age. I can see no more
difficulty in this than in the unhatched young of other birds acquiring the
instinct to break through their own shells; or than in young snakes
acquiring in their upper jaws, as Owen has remarked, a transitory sharp
tooth for cutting through the tough egg-shell. For if each part is liable
to individual variations at all ages, and the variations tend to be
inherited at a corresponding or earlier age--propositions which cannot be
disputed--then the instincts and structure of the young could be slowly
modified as surely as those of the adult; and both cases must stand or fall
together with the whole theory of natural selection.
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