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 F, 
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 F,. 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 F, 
generation has ears of intermediate length and that 
the blend is ‘‘permanent,”’ 7.e., that “no reappearance 
of the grandparental ear length occurs in generation 
F., 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 F,. 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 
