C BOWMAN LECTUEE. 



the normal can perhaps hardly yet be either asserted or 

 denied. Until more, and more precise, data are collected, 

 we cannot tell whether the order of the birth of the 

 children, or the age of the parents at marriage, have any 

 influence in determining the disease. The relative fre- 

 quency with which the same fundamental cause produces 

 retinitis pigmentosa in one, deafness in another, and 

 mental inferiority in a third member of the same pedi- 

 gree, has not yet been worked out ; but we find on the 

 other hand that certain stocks produce only retinitis pig- 

 mentosa and others only the equivalent deafness. It is 

 especially noteworthy that the largest pedigrees of the 

 retinitis are quite free from the other degeneracies, and 

 the survival of such families is probably due to this cir- 

 cumstance. 



I should like to return for a few moments to the two 

 kinds of descent, continuous and discontinuous, met with 

 in this disease. As I said in my introductory remarks, 

 continuous descent in Mendel ian terminology usually 

 means " dominance," and interrupted descent, except in 

 sex-limited conditions, means " recessiveness." Retinitis 

 pigmentosa, although more frequent in the male, cannot be 

 put into the sex-limited class with colour-blindness, Leber's 

 disease and others in which women very seldom suffer. 

 Therefore since, as Figs. 36 and 37 show, pedigrees exist 

 in which a healthy generation always comes in the direct 

 line between two that contain retinitis pigmentosa, or 

 one of its equivalents, and since the normal c: carrier " 

 may be of either sex, the disease must then, in Mendel's 

 terms, be recessive."* And yet in the largest pedigrees the 

 descent, as I have already said, is always continuous and 

 the disease therefore dominant. 



This is as far as we can go at present. In the discon- 

 tinuous pedigrees we can make sure that the intervening 

 generation has neither eye disease, deafness nor mental 

 defect ; but there may perhaps be other morbid states, 

 other equivalents of retinitis pigmentosa, that give no con- 

 * For the data see Appendix I d. 



