176 MENDELISM CHAP. 



unknown, in the female. Sex-limited inheritance of a 

 similar nature is known for one or two ocular defects, and 

 for several diseases of the nervous system. In the pe- 

 culiarly male disease known as haemophilia the blood re- 

 fuses to clot when shed, and there is nothing to prevent 

 great loss from even a superficial scratch. In its general 

 trend the inheritance of haemophilia is not unlike that of 

 horns among sheep, and it is possible that we are here 

 again dealing with a character which is dominant in one 

 sex and recessive in the other. But the evidence so far 

 collected points to a difference somewhere, for in haemo- 

 philic families the affected males, instead of being equal 

 in number to the unaffected, show a considerable prepon- 

 derance. The unfortunate nature of the defect, however, 

 forces us to rely for our interpretation almost entirely 

 upon the families produced by the unaffected females 

 who can transmit it. Our knowledge of the offspring of 

 " bleeding "males is as yet far too scanty, and until it is 

 improved, or until we can find some parallel case in ani- 

 mals or plants, the precise scheme of inheritance for 

 haemophilia must remain undecided. 



Though by far the greater part of the human evidence 

 relates to abnormal or diseased conditions, a start has 

 been made in obtaining pedigrees of normal characters. 

 From the ease with which it can be observed, it was 

 natural that eye-colour should be early selected as a 

 subject of investigation, and the work of Hurst and others 



