BOWMAN LECTURE. 



XCV 



elsewhere,* and to-day I need only allude to some of the 

 principal points and ask attention to some of the unsettled 

 problems in the natural history of this disease and its 

 equivalents. I will keep almost clear of numbers, but may 

 mention that the paper referred to was based upon notes of 

 nearly 1000 families (strictly 976) containing an average 

 of close upon two ascertained cases of the disease in each. 

 There was proof of heredity in one quarter of the families 

 and of consanguinity of parents or ancestors of those 

 affected in another quarter (I use round numbers, the 



exact figures are given in the original paper). In the 

 remaining half, where no history of either consanguinity 

 or heredity was recorded, the notes were often very imper- 

 fect, and there can be no doubt that such a history would 

 often have been forthcoming had more pains been taken. 

 In the largest pedigreest of retinitis pigmentosa the 

 descent of the disease is continuous from parent to child, 

 no healthy member ever producing affected offspring 

 (Fig. 35). 



* Nettleship, R.L.O.H., xvii. 



f To the fully recorded pedigrees quoted in the paper above referred 

 to Snell has since added another in T.O.S., xxvii, 1907, p. 217. 



