HEREDITY 



hygiene and bad hygiene. And the alterations to be expected 

 in man by environmental diversity are probably less than 

 for any other organism, owing to the delicacy of his 

 organization and the mechanisms that have been produced 

 to keep his environment constant. 



I know of no better analogy to use in comparing the 

 respective functions of heredity and environment than one 

 borrowed from photography. The gene plan of the fertilized 

 egg makes it like an exposed plate. The potentiality of a 

 picture is there, waiting to be developed. The environment 

 is the developer. It can make or mar the picture, but that 

 picture will h^ve the same general character in any event. 

 Translating these matters into the terms of sociology, one 

 may say: Give the growing child the best conditions possi- 

 ble; it is highly desirable; but do not expect to change the 

 character ofhis features or the quality of his brains. 



If it has not been made perfectly clear before, the last 

 sentence will serve to indicate that man cannot be excluded 

 from the aggregation of organisms ruled by genetic law. 

 Well, why should he be excluded? Man is simply a higher 

 mammal, with a structure and a set of physiological 

 processes very similar to those of other mammals high in 

 the evolutionary series. From these facts one might assume 

 that the method by which his heritage is passed on would 

 be essentially the same as that which is common to all the 

 remaining members of the animal and vegetable kingdoms 

 which reproduce sexually. But no such abstract deduction 

 is necessary. The observations of competent cytologists 

 have shown conclusively that human body cells contain 

 forty-eight chromosomes, and that, at the maturation of 

 the germ cells, these chromosomes undergo the special type 

 of reduction division required for the distribution of the 

 genes by the Chromosome Theory of Heredity. In addition, 

 analyses of voluminous genealogical records have demon- 

 strated that numerous character differences, affecting each 



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