THE MENDELIAN CLUE 233 



represented by differences in the ultra-microscopic 

 architecture. It is quite likely that several factors 

 may be concerned in one character, or that one factor 

 may influence more than one character. 2. The 

 second idea in Mendelism is that of dominance. 

 When Mendel crossed a pure-bred tall pea with a 

 pure-bred dwarf pea, the offspring were all tall ; and 

 he called the quality of tallness dominant to the 

 recessive quality of dwarfness, which the hybrid 

 offspring keep, as it were, up their sleeve. The 

 dwarfness is not expressed, but it is certainly in the 

 inheritance, for it reappears in a quarter of the 

 progeny of the hybrid generation, if these are inbred 

 or allowed to self-fertilize. If a Japanese waltzing 

 mouse is crossed with a normal mouse, all the hybrid 

 progeny are normal, the waltzing peculiarity being 

 recessive to normality. If the hybrids be inbred, 

 some of their progeny are waltzers in the average 

 proportion of a quarter and these waltzers might 

 be sold as pure waltzers although both their parents 

 and one of their grandparents were normal. Simi- 

 larly, about a third of the rest of the progeny are 

 purely normal, while two-thirds are like the first 

 generation of hybrids to all appearance normal, but 

 with the waltzing character up their sleeve. It often 

 happens that the two parents differ, not in presenting 

 a pair (or more) of contrasted or alternative char- 

 acters, but in the one having certain unit characters 

 which the other has not. This works out in the 

 same way the unit character that is present being, 

 as it were, dominant to its own absence. In 



