OUE NATURAL INHEEITANCE 67 



In crosses between long-eared lop rabbits and a short- 

 eared breed, Professor Castle found that the offspring 

 were intermediate in length of ears and in a number 

 of skeletal measurements. Many plant hybrids are 

 very exact blends of their parents, and the same mode 

 has been well illustrated in crossing two cockatoos 

 belonging to different genera. 



(d) Mendelian inheritance has to do with unit-char- 

 acters and their distribution in the progeny. If a 

 normal mouse is crossed with a Japanese waltzing 

 mouse, all the hybrid offspring, (first filial generation 

 Fi), are normal. Normal behaviour or its factor is said 

 to be dominant as compared with waltzing behaviour 

 or its factor, which is spoken of as recessive. If the 

 hybrids be inbred, or bred with others of similar lineage, 

 about a quarter of the progeny are waltzing mice and 

 these waltzers might be sold as * pure ' waltzers — • 

 although both their parents and one of their grandparents 

 showed the normal behaviour. The rest of the progeny 

 in this second filial generation (Fg) appear to be quite 

 normal, but experiment shows that about a third of 

 them are ' pure ' normals, while about two-thirds are 

 like the hybrid generation Fi, with every appearance 

 of being normal, but with the waltzing character latent 

 — up the animal's sleeve, as it were. If the pure 

 normals, or, as they are technically called, " pure 

 dominants," are bred together or with others like 

 themselves, they will yield again the proportions 

 observed in the second filial generation, namely, 25 per 

 cent, pure normals plus 50 per cent, impure normals 



