CHAPTER XIX 



HEREDITY 



1. The facts of heredity 2. Theories of heredity historical retro- 

 spect 3. The idea of the continuity of generations 4. Modes 

 of inheritance 5. Social and ethical aspects. 



IN olden times men spoke of the three Fates which were 

 believed to determine of what sort a life should be. With 

 the decay of poetic feeling, and in the light of common 

 science, the forms of the three sisters have faded. But 

 they are realities still, for men are thinking more and 

 more vividly about the factors of life. The biological 

 factors are Heredity, Function, and Environment : the 

 capital with which a life begins, the interest accruing 

 from the investment of this in varied vital activities, and 

 the force of circumstances. But while it is useful to think 

 of Heredity, Function, and Environment as the three 

 Fates, we must not mystify matters by talking as if these 

 were entities acting upon the organism. They are simply 

 aspects of the fact that the creature is born and lives. 

 The inheritance includes all that the organism is or has 

 to start with when it begins its life as a fertilised egg- 



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cell, and heredity is only a name for the genetic relation 

 between successive generations. Moreover, the functions 

 of an organism depend upon the inherited and the 

 acquired constitution, and so does its susceptibility to 

 the influences of environment. Function includes all 

 the actions and reactions between the living creature 



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and its surroundings, and its influence may operate 

 through disuse as well as through exercise. 



We define heredity as the relation of organic or genetic 



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