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 eggshell. 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.