INHERITANCE OF MENTAL DEFECTS AND DISEASE 37 



into clearly defined classes. This fact does not necessarily indi- 

 cate, as Pearson and Heron imply, that the various kinds of men- 

 tal defect are not transmitted according to Mendel's law. It is 

 not uncommon for segregation to occur in the usual Mendelian 

 manner, although the character segregated may fluctuate so as to 

 form a perfectly continuous series. Where the germinal factors 

 manifest themselves somatically in characters that undergo a 

 large amount of fluctuating variability, it naturally makes the 

 demonstration of Mendelian segregation more difficult. Where, 

 as in human beings, it is not feasible to employ experimental 

 methods of analysis the difficulty of establishing Mendelian 

 inheritance beyond cavil is greatly enhanced. One has to be 

 guided by probabilities. The best that can be done is to select 

 tentatively that hypothesis which gives the most plausible inter- 

 pretation of the phenomena to be explained and is best in accord 

 with what is known of the principles of inheritance followed in 

 other fields. The very general occurrence of Mendelian inheri- 

 tance among plants and animals of both primitive and highly 

 organized types, and the remarkable success attained in explain- 

 ing apparently non-conformable phenomena in terms of Mendel's 

 law, creates a very justifiable presumption in favor of the conclu- 

 sion that mental defects are transmitted according to the same 

 laws that prevail so widely in the plant and animal world. That 

 inheritance in man obeys the laws followed by organisms in gen- 

 eral is also indicated by the undoubted appearance of types of 

 Mendelian inheritance among human characteristics. 



But while the general occurrence of Mendelian inheritance in 

 the organic world creates a presumption in favor of the conclusion 

 that mental traits in man are transmitted according to the same 

 rule, it must be conceded that there are certain characters whose 

 mode of transmission seems to present a clear exception to this 

 type of inheritance. It is true that such cases are comparatively 

 rare. But there is a much larger number of cases which may 

 follow Mendel's law, but in which it has never been proven that 

 they actually do follow it. The successful extension of Mendelian 

 analysis may justify us in shifting the burden of proof from the 



