114 HEREDITY 



of circumstances, and does not depend upon a 

 primary nervous defect, there is little reason to 

 believe that this acquirement is often transmitted; 

 though even Weismann admits the possibility that 

 persistent nerve fatigue may affect the germ-plasm, 

 and so give rise to neurotic tendencies in the off- 

 s-pring. The subject is too large to be dealt with 

 here. As to the transmission of insanity, this may 

 be added to what has already been said, that " there 

 is no clear case of a normal subject becoming an 

 acute maniac through external shock and trans- 

 mitting his disease." 



Lastly, as to those definite specific diseases which 

 are unquestionably transmitted. These are of 

 germinal origin. Highly typical, and of very great 

 interest in relation to the discoveries of Mendel/ 

 are the " bleeding disease," or haemophilia, and 

 colour-blindness. These usually show themselves 

 in males, but are transmitted by females. For 

 instance, a colour-blind man had two normal 

 daughters, each of whom had one colour-blind son, 

 whose three normal daughters had four colour-blind 

 sons, and so on, until in the sixth generation there 

 were eight males, of whom seven were colour-blind. 

 Similar genealogical trees, in cases of both of these 

 diseases, might be multiplied without limit. Abnor- 

 malities, such as the occurrence of six fingers or 

 toes, may also be traced through as many as half-a- 

 dozen generations. 



1 See Chapter VIL 



