HEREDITY 



undergo only such changes as are permitted by the gene 

 pattern; and these changes are limited in type and degree. 

 An experimenter can modify the chemical constitution and 

 the temperature of sea water quite materially and thus 

 obtain bizarre fishes having Cyclopean eyes or two heads 

 or divided tails; but these monstrosities are not chaotic 

 expressions of change in every direction; on the contrary, 

 they exhibit the natural potentialities of the gene patterns 

 under the novel conditions presented. We need not dwell, 

 however, on the character, degree, or frequency of the 

 variations produced in such special cases. The matter to be 

 emphasized, it seems to me, is that such animals as the 

 higher mammals cannot have their environmental condi- 

 tions altered materially and live. Just consider for a moment 

 the extreme nicety of man's regulatory apparatus. Tem- 

 perature is controlled; moisture is regulated; and the types 

 of food permitted are severely restricted. If his temperature 

 rises half a degree, he is ill. If he does not get a balanced 

 ration consisting only of fats, carbohydrates, proteids, 

 essential minerals and vitamines, growth is prohibited. He 

 cannot even synthesize his own protoplasm unless he has 

 all the necessary amino-acids. And if his blood absorbs a 

 tiny fraction of any one of a million different chemicals, he 

 dies. One should not expect to find great plasticity under 

 such conditions. Moreover, man is a delicate machine in 

 another sense. When developmental errors occur and 

 monstrosities are formed, these deviants are seldom viable. 

 Generally speaking, therefore, the effect of any ordinary 

 change of environment is negligible. It is usually confined 

 to the production of well-nourished or ill-nourished off- 

 spring; and this is true, whether one is speaking of plants 

 or animals, of unspecialized forms or highly specialized 

 forms. But it should not be forgotten that even in man 

 himself an occasional serious aberration is attributable to 

 the environment. An accident may prevent normal matu- 



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