LX BOWMAN LECTURE. 



that all his children will, as a rule, have normal colour- 

 perception ; that if his sons, who neither exhibit nor carry 

 the defect, have issue, that issue too will be normal ; but 

 that some of the sons of some of his daughters will show 

 the defect, whilst the other daughters, who we presume do 

 not carry it, will have all normal children. If one of these 

 normal children of a normal daughter marries a cousin, 



A i * -n t 1 1 



x (J mo.lt w .Disuse inusTmTecV 

 IMormal / 



O female each 



(J O Twins. O .Died 



Vvlith ov Without sex slftnS ; Two or more 



s^\ 

 vx 



the (lumber Sometfrnes 



^ OtKer Diseases ^Kan tKe 



on \\vxstro.ted b the Order of ii'tth 



09 



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o"t thexe 



the issue of one of the normal sons, the result, as regards 

 colour-perception, will be the same as if two unrelated 

 normals marry. In fact, if the sex-limitation were in- 

 variable in colour-blindness and other sex-limited con- 

 ditions, only one kind of cousin-marriage out of the 

 several possible kinds shown in Figs. 1 to 5 (I speak of 

 first cousinship throughout) would be attended by special 

 risk, i'Vz., when the mothers of the parents are sisters who, 



