'^'2.2 A STUDY OF HEREDITY IN INSANITY [Oct. 



seemed to be no regularity in the working of heredity, and the 

 generally accepted conclusion on the subject has been well voiced 

 by Kraepelin : '" We must therefore regard the statistics of 

 heredity in insanity merely as facts of experience without finding 

 in them the expression of a ' law ' which should hold in every 

 case." ' 



In recent years, however, it has been shown that human 

 heredity, at least as far as certain traits are concerned, is subject 

 to general biological laws. Special mention may be made of 

 color of eyes,' color of hair,* form of hair,° brachydactyly,' some 

 forms of cataract,' and retinitis pigmentosa,* as human traits 

 which have been shown to be transmitted from generation to 

 generation in accordance with the Mendelian theory. 



As regards insanity and allied neuropathic conditions, the facts 

 to which we have already referred, namely, the facts of atavistic 

 and collateral heredity, direct heredity, and the frequent failure 

 of transmission seem to point plainly to alternative inheritance. 

 This suggests the likelihood of a mechanism of inheritance 

 according with the Mendelian theory, and the present study has 

 been undertaken'with a view to determining whether indeed the 

 neuropathic constitution is transmitted in the manner of a Men- 

 delian trait. 



§ I. The Mendelian Theory." 



Perhaps a brief statement of the Mendelian theory will not be 

 out of place here. 



The total inheritance of an individual is divisible into unit 

 characters, each of which is, as a general rule, inherited inde- 

 pendently of all other characters and may therefore be studied 

 without reference to them. 



^ Loc. cit. 



'Davenport, Science, N. S., Vol. XXVI, Nov. i, 1907. Hurst, Proc. 

 Roy. Soc, Vol. LXXX B., 1908. 

 ■* Davenport, The Amer. Naturalist, Vol. XLIII, .\pr., 1909. 



* Davenport, Loc. cit., Vol. XLII, May, 1908. 



* Farabee, Papers of Pcabody Museum, III, 3, 1905. 



' Nettleship, Rep. Roy. London Ophth. Hosp., XVI, 1905. 

 'Nettleship, Loc. cit., XVII, Parts I, II, and III. 



* This section is for the most part reprinted from the Preliminary 

 Report made shortly after the present study was begun. (Cannon and 

 Rosanoff. Jour, of Nerv. and Mcnt. Disease, May, 191 1.) 



