H. H. Newman 37 



Nettleship with respect to rayopia, and I find strabismus^ almost as 

 invariable an accompaniment of night-blindness. An expert ophthalmo- 

 logist would doubtless detect several other well-defined affections, which 

 almost certainly are present in those cases where a near approach to 

 complete blindness is described, for, as I understand it, complete 

 blindness is never the result of either night-blindness or any of the 

 associated conditions known to exist in this family. 



The frequent occurrence in the pedigree of the minor defect 

 Pterygium'^ seems to be somewhat unusual. The defect is evidently 

 strongly inherited, probably as a Mendelian dominant, since it is neither 

 sex-limited nor necessarily associated with the recessive night-blind 

 complex. Peculiarly enough, however, although not strictly sex-limited, 

 it occurs more commonly and assumes a moie serious aspect in males 

 than in females. 



It would appear then that night-blindness, myopia, strabismus (and 

 possibly some other conditions that have not been diagnosed) form a 

 complex due to a single sex-linked factor and that whether one, two 

 or all of the conditions appear simultaneously or successively in an 

 individual may depend upon cooperating or counteracting factors in 

 the germ plasm of the normal parent, producing an intensification or a 

 dilution of the somatic expression of the germinal defect. 



In conclusion it may be well to point out that, if we follow Wilson's 

 suggestion, it is no longer admissible to interpret such conditions as 

 colour-blindness and this type of night-blindness as due to a factor 

 dominant in males and recessive in females, for it seems pretty well 

 established that such characters are really recessive and only express 

 themselves, after the manner of recessives, in the absence of the 

 dominant allelomorph. 



1 Nettleship suggests that strabismus is due to the necessity the affected are under 

 of bringing the book, etc. very close to the eyes, and is unrelated with night-blindness as 

 such. 



2 Nettleship is of the opinion that Pterygium as such is not inherited but that there is 

 merely a congenital peculiarity of the mucous membrane predisposing to Pterygium, The 

 defect itself is commonest in people whose eyes are constantly exposed to glare, dust and 

 irritants over a number of years. The glare and dust of Texas are characteristic elements 

 of the climate. 



