186 THE MENDEL JOURNAL 



stocks which manifest more or less obvious degeneracy. 

 Mr. Nettleship regards retinitis pigmentosa as itself a 

 degenerac}'.* The degeneracy in the stock ma}^ take 

 various forms. It may not always be retinitis pigmentosa 

 or lenticular cataract. We may have three or four 

 generations without these specific diseases showing 

 themselt^es, and then apparently quite suddenly they 

 appear. But an examination of many of the pedigrees 

 of retinitis pigmentosa reveals the existence of other 

 degeneracies, such as deafness of various degrees, and 

 when complete associated with dumbness ; of congenital 

 idioc}^ many varieties of mental deficiency, insanit}', 

 epilepsy, and progressive paralysis ; of polydactylism, 

 remains of the hyaloid artery, congenital cataract, 

 conical cornea, and coloboma iridis.l 



What is the meaning of this association between 

 certain eye defects and these degenerate traits? It is too 

 common to permit us to dismiss it as an accidental or 

 coincidental phenomenon. We believe the ultimate 

 explanation will be found in somatic segregation. Gametes 

 may carry into the zygote the same group of factors and 

 yet the zygotic manifestation may be very different in 

 different cases. 



Let us take for instance the case of deafness. What 

 is deafness ? It ma}^ be produced by a variety of causes. 

 We may leave out of consideration the cases of middle-ear 

 disease and confine ourselves to physiological defects or 

 structural imperfections of the internal ear and of the 

 auditory centre in the brain. Now it is conceivable that 

 deafness may be produced in one of two ways. In the 

 first, some essential group of nerve cells in the brain or 

 in the auditory epithelium may be absent. In the 

 second place, these nerve cells may all be present and yet 

 their physiological activities may be inhibited by the 

 presence of some inhibiting factor. Let us deal with the 

 second consideration. Suppose in some cases, that in 

 the course of somatic cell divisions, this inhibitory factor 

 became part of the nuclear contents of the cells of the 

 auditor}^ centre in the brain, or of the cells of the auditory 

 epithelium. The individual would then be deaf. 

 Suppose, however, that in the course of these somatic cell 



* Op. cit. p. 342. t Op. cit. p. 343. 



