272 HEREDITY AND DISEASE 



§ 6. Particular Cases 



Colour-blindness. — This peculiar condition may recur for many- 

 generations, but it has an interesting peculiarity. It is usually 

 restricted to the male members, yet a colour-blind man seems 

 never to have a colour-blind son unless the peculiarity was also 

 in his wife's family. Colour-blindness is transmitted from father 

 to grandson through unaffected daughters. 



Short-sightedness. — It is generally admitted that short- 

 sightedness is due to an inborn peculiarity in the structure of 

 the eye, occurring in various degrees. In itself it can hardly 

 be called a disease in the strict sense, and conditions of life are 

 conceivable in which it might even be advantageous. The 

 innate peculiarity may become exaggerated and complicated 

 when the eyes are forced to function in a way to which they 

 are ill adapted, and acquired " myopic " modifications may 

 be superadded to what was there by inheritance. Sometimes 

 these may even lead to an actually diseased condition. But 

 though the innate peculiarity may be exaggerated and 

 complicated by the addition of acquired modifications, 

 there is no evidence that these can be transmitted. What 

 is transmitted is a structural peculiarity which began as a 

 germinal variation, and that this is very liable to be trans- 

 mitted one does not require to go to Germany to see. 

 It is said that short-sightedness occurs, though rarely, among 

 wild races. 



Bleeding.v— A hemorrhagic tendency or liability to bleeding 

 is well known to be heritable, but it finds expression only in 

 males. A case given by Klebs and cited by Sir William Turner 

 (1889) is instructive in showing how the tendency, though 

 transmitted through daughters (and therefore part of their 

 inheritance), finds expression only in the males, and in 

 illustrating first a diffusion, and then a waning of the peculi- 



