LENTICULAR CATARACT 185 



individuals. On that basis the phenomena can be 

 brought into Hne with MendeHan principles. The 

 existence of characters which depend upon the simul- 

 taneous operation of two or more factors is not an idle 

 hypothesis. It is a practicall}^ demonstrated fact in the 

 coat colour of animals and the petal colour of flowers. 

 In peas, for instance, we know of actual cases where 

 coloured parents will produce white-flowered members 

 among their offspring in addition to coloured individuals. 

 There we have the analogy of the appearance of a mixed 

 offspring in the progen^^ of parents, one or both of whom 

 are affected with lenticular cataract or retinitis pig- 

 mentosa. On the other hand, we can seek the analogy 

 for the appearance of affected members from parents 

 unaffected with either of these diseases, in the crossing 

 of the white races of Emily Henderson sweet peas, which 

 give coloured individuals among their offspring. But it 

 has been shown that though these two races are identical 

 in having white flowers, they are really two different 

 races. One is characterised by having round pollen- 

 grains and the other by oval pollen-grains. If the 

 members of the oval-pollened race are self-fertilised or 

 fertilised inter se, only white-flowered offspring result. 

 And similarly for the round-pollened race. But if the 

 round-pollened white race be crossed with the oval- 

 pollened white race, some coloured members are produced 

 among their offspring. 



Now if we knew nothing of the existence of characters 

 dependant upon the simultaneous presence in the body 

 of the same individual of two determining factors, the 

 production of coloured-flowered plants in the offspring 

 of two white parents could not be interpreted on Mendelian 

 lines. There is no reason to doubt that many human 

 qualities and characters depend upon the co-ordinate 

 action and simultaneous presence of several factors. 

 Eliminate one only of the many factors required for the 

 normal manifestation of such a human character and 

 abnormality of greater or less degree will be exhibited. 

 It matters little how that elimination may be brought 

 about ; it may be gametic or zygotic in origin. Now it is 

 a significant fact that many of these family histories of 

 retinitis pigmentosa and lenticular cataract belong to 



