ORGANIC EVOLUTION— MENTAL 161 



new environment a greater divergence in mental traits 

 from their ancestors than animals in whom instinct 

 predominates. That this is true we have ample evi- 

 dence. 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 ai-e 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 develop- 

 ment 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 con- 

 dition, or greatly alter it, by withholding or by altering the 



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