BOWMAN LECTURE. CI 



spicuous signs, and at present, therefore, escape detection. 

 This is mere speculation for future work ; the arteriole 

 disease leading to retinitis pigmentosa, or to deafness or 

 mental deficiency, may possibly in some cases affect an 

 entirely different region, e. g. the arterioles of kidneys or 

 liver or even of the hands or feet. But at present, if we are 

 to test our data for retinitis pigmentosa by the Mendelian 

 scheme, we must assume that change in mode of descent 

 means change of dominance, however improbable this may 

 appear. We were formerly content to say that a given 

 disease or character could become latent for a generation 

 or more and then re-appear, either capriciously, or perhaps 

 when re-inforced by a marriage between cousins. But 

 the Mendelian conception of pairs of complementary 

 characters, one of which, in virtue of some attribute, domi- 

 nates or prevents the appearance of the other, does not 

 in its simple form allow the dominant to lose dominance or 

 the recessive to gain it. But if the members of a pair 

 representing a given character, say retinitis pigmentosa 

 and its absence, could, without losing their affinity, become 

 linked with, and influenced by, a pair representing some 

 other character, a change of dominance in the original 

 pair might conceivably be brought about, the second or 

 linked character not necessarily attracting attention.* 

 This is only the crudest possible indication of the ingenious 

 hypothesis of " coupling," by which some of the complex 

 and unexpected results obtained in experimental breeding 

 are explained, and which appears to have been verified 

 by control experiments in certain cases. 



Of the varieties of retinitis pigmentosa, retinitis punc- 

 tata albescent has, so far as I know, never been seen in 

 a well-marked form in more than one generation, and if 

 it is not a new departure, a " mutation," it must, in some 

 cases, have skipped several generations. 



* Of. Lock, R. H., " On the Inheritance of Certain Invisible Characters 

 in Peas," Proc. Roy. Soc., Ixxix B., JC07, p. 28. 



