224 Colour-Blindness [ch. 



According- to the simple plan thus sketched, all the sons 

 of the affected females will have the affection and exhibit it. 



The only one of the notoriously sex-limited conditions 

 which is available for testing this rule is Colour-blindness. 

 Haemophilia and Gowers' disease are too fatal, and night- 

 blindness is too rare and too little known. Regarding Colour- 

 blindness even, the evidence is not enough yet to provide a 

 statement as to the quantitative results. A search undertaken 

 by Mr Nettleship has however been successful in so far that 

 he has obtained evidence of five colour-blind women with 

 sons, in all eleven, who are all colour-blind. In three cases 

 also there is, in addition, evidence that the women's fathers 

 were colour-blind, as required by the scheme ; and in one 

 case also there is a record that the woman came from a 

 strain having colour-blind members. Besides these there 

 are two published accounts of families of two colour-blind 

 women, each with three sons, all colour-blind. In all there- 

 fore the seven colour-blind women had ly sons, all colour- 

 blind. We can I think feel no further doubt that the 

 scheme must so far present an approximation to the actual 

 facts. In dealing with such phenomena exceptional cases 

 must be expected, but hitherto they have not been found ''\ 



Colour-blindness is not, therefore, as might have been 

 imagined, a condition due to the omission of something 

 from the total ingredients of the body, but is plainly the 

 consequence of the addition of some factor absent from 

 the normal. We can scarcely avoid the surmise that this 

 added element has the power of paralysing the colour-sense, 

 somewhat as nicotln-poisoning may do. 



The other sex-limited diseases in all probability follow 

 similar rules, but in regard to haemophilia there are 

 various difficulties. The records are too heterogfeneous 

 for satisfactory tabulation as yet, and it is to be suspected 

 that more than one condition may pass by the same name. 

 One remarkable feature must be mentioned, namely that 

 the records show with great constancy that too many males 

 are affected and too many females transmit, the excess being 

 far greater than any which could readily be ascribed to 

 recorders' errors. This excess is also strongly exemplified 



* By the kindness of Miss J. E. Downey of Wyoming University I 

 have since been informed of a case which is probably a real exception — a 

 woman with defective colour-sense having; a child with normal colour-vision. 



