CHAPTER XV 



MAN 



THOUGH the interest attaching to heredity in man 

 is more widespread than in other animals, it is far 

 more difficult to obtain evidence that is both com- 

 plete and accurate. The species is one in which 

 the differentiating characters separating individual 

 from individual are very numerous, while the number 

 of the offspring is comparatively few, and the gener- 

 ations are far between. For these reasons, even if 

 it were possible, direct experimental work with man 

 would be likely to prove both tedious and expensive. 

 There is, however, another method besides the direct 

 one from which something can be learned. This 

 consists in collecting all the evidence possible, ar- 

 ranging it in the form of pedigrees, and comparing 

 it with standard cases already worked out in animals 

 and plants. In this way it has been possible to 

 demonstrate in man the existence of several char- 

 acters showing simple Mendelian inheritance. As 

 few besides medical men have hitherto been con- 

 cerned practically with heredity, such records as 

 exist are, for the most part, records of deformity or 

 of disease. So it happens that most of the pedigrees 

 at present available deal with characters which are 



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