CHAPTER XX 



THE INFLUENCE OF FUNCTION AND ENVIRONMENT 



1. The influence of function 2. The influence of surroundings 

 3. The transmissibility of acquired characters or somatic 

 modifications 4. The importance of nurture 5. The other 

 side of heredity. 



1. The Influence of Function. A skilled observer can 

 often discern a man's occupation from his physiognomy, 

 his shoulders, or his hands. In some unhealthy occupa- 

 tions the death-rate is three times that in others. Disuse 

 of such organs as muscles tends to their degeneration, 

 while increased exercise is within certain limits associated 

 with increased development. 



Precise illustrations of the influence of function on 

 animals are far from being abundant. Three sample 

 cases may be cited. (1) A Japanese investigator, Shin- 

 kishi Hatai, has shown .in the case of the white rat that 

 long-continued exercise markedly increases the weight 

 of the heart, kidneys, and liver, on an average to about 

 20 per cent. He exercised the rats for 90-180 days, 

 which is comparable to a period of 7-14 years in man, 

 for the span of life in the white rat is about three years. 

 (2) Semper and De Varigny found that when freshwater 

 snails were reared in vessels of a shape that allowed them 

 abundant water but very little surface on which to 

 take exercise they developed into dwarf forms. Every 

 precaution was taken to secure abundant food, perfect 

 aeration, and thorough removal of waste-products. 

 De Varigny's experiments were particularly careful 

 and point convincingly to the conclusion that the con- 



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