234 MULTIPLE FACTORS 



birds are backcrossed (Fig. 57) to the fantail the 

 number of feathers varies from 19 to 31. On the 

 hypothesis that the race of fantails has been built 

 up by the accumulation of several factors these results 

 can be understood. 



MacDowell has compared the length of skull and 

 of one of the bones in the leg (ulna) of hybrids be- 

 tween domesticated races of rabbits in the Fi 

 generation and in the backcross. As shown in the 

 table, page 185, the variability of the backcross is in 

 both characters greater than that of Fi. Similar 

 though less convincing evidence was obtained for 

 body weight also. 



The inheritance of ear length in rabbits has been 

 studied by Castle in a cross between lop-eared and 

 short-eared races (Fig. 58). He shows that the Fi 

 generation has ears of intermediate length and that 

 the blend is "permanent," i.e., that "no reappearance 

 of the grandparental ear length occurs in generation 

 F2, nor are the individuals of the second generation, 

 as a rule, more variable than those of the first 

 generation of cross breeds.'' In the light of Mac- 

 Dowell's results for other quantitative characters in 

 rabbits it seems more probable that the number of 

 factors involved is greater for ear length than in the 

 other cases, hence more data will be necessary before 

 we can be certain that no reappearance of the 

 grandparental types will be found in F2. If four 

 independent factors were involved either grand- 

 parental type would be expected to reappear only 

 once in 256 times, with six factors only once in 4096 



