new environment a greater divergence in mental traits from their ancestors than animals in whom instinct predominates. That this is true we have ample evidence. For example, a chicken or lamb, removed from the care of its mother and reared by hand, differs mentally more from the rest of its species than does a fish or a frog similarly reared, but not nearly so much as a parrot, jackdaw, or puppy, and immeasurably less than does an infant reared even by members of its own species, but of a different race, e. g. a child of savage parentage reared by civilized people. A fish, which starts life with such a large equipment of instinct, can never be tamed to the same degree as a chick or a lamb, which starts life with a smaller equipment; a chick or a lamb can never be taught the many things that a jackdaw or a dog are capable of learning; as to the extraordinary power of acquiring mental traits possessed by the human infant, which starts life with such a small equipment of instinct, it need not be dwelt on, as it is known to all.
Mental traits, like physical traits, when once acquired, are more or less persistent. By withholding appropriate stimulation, that of use, from the limb of an infant, e.g. by paralyzing it, we are able to prevent its development into an adult limb; so, by withholding appropriate stimulation from the mind of an infant, we are able to keep it in a more or less infantile condition, as has happened in the case of various individuals incarcerated from infancy in Eastern prisons for political reasons; but as, when once a corporal structure has attained its full development, we cannot reduce it to its infantile condition by withholding stimulation, or greatly alter it by altering the kind of stimulation; so, when once a mind, whether human or brute, has attained its full development, we cannot reduce it to an infantile condition, or greatly alter it, by withholding or by altering the
