CHAPTER XX 

 HEREDITY IN MAN 



The study of heredity in the human race is hindered in a number 

 of ways. Because of the span of generations, results approximating 

 those secured in the laboratory in the study of other organisms 

 are impossible. It is rare to hear of five generations or even four 

 aUve at the same time in a human family, and most of us never 

 know more than three, so it is necessary to fall back upon records 

 and these are at present pitifully incomplete. Genealogies furnish 

 some valuable data, the records of pul^lic institutions are also a 

 dependa])le source of information, and within the present century 

 such institutions as the Eugenics Record Office at Cold Spring 

 Harbor, Long Island, have begun the work of making accurate 

 scientific records in this field. 



Even the accurate observation and recording of natural phe- 

 nomena cannot, however, give results like those obtained in the 

 study of laboratory animals. It is and will probably always be 

 impossible to control the reproduction of human beings except 

 in extreme cases which demand the action of organized society 

 for its own protection. Fortunately such methods, undesirable 

 from the normal human viewpoint, are not essential to an under- 

 standing of human inheritance. We are animals and there is every 

 reason to believe that the laws of heredity in other organisms are 

 equally applicable to ourselves. The corroboration of this rela- 

 tionship by the available data is adequate. 



One unfortunate feature of our knowledge of human heredity 

 is that extreme cases and particularly abnormalities are most 

 likely to make an individual the object of scrutiny. Data are 

 more abundant concerning the inheritance of supernumerary digits 

 and mental defects than on the behaviour of valuable qualities, 

 although the latter are by no means lacking. Fortunately accurate 

 data, whatever the characters recorded, are a valuable basis for 

 the application of general laws as worked out in other organisms, 

 and at least a partial indication of the trend of heredity in general. 



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