256 
GENETICS: W. E. CASTLE 
cabin of a native in Arequipa, Peru, this Arequipa guinea-pig being of 
a size about one half greater than that of race B males. The growth 
curve of race B males is shown in figure 4 where it can be compared with 
the growth curves of Fi and F2 male animals obtained by crosses of the 
Arequipa male with race B females. It will be observed that the growth 
curve of the Fi males runs a course in general parallel with that of the 
growth curve for race B males but at a much higher level. The growth 
curve of the F2 males starts in at a still higher level, perhaps in part 
because the vigorous Fi mothers supply an abundance of nourishment 
to the young. But from weaning time on (age about 20-30 days) the 
F2 males grow less rapidly. Their curve crosses the Fi curve at 40 
days and continues thence below it so that its ultimate position is 
Weight 
Orams 
Age in Days 40 
FIG. 3. GROWTH CURVES OF RACE B AND CUTLERI MALES AND OF THEIR MALE HYBRIDS 
BOTH Fi AND F2. 
intermediate between the Fi and the race B growth curves. It is evident 
that here, as in the Cutleri crosses, the Fi animals are larger than 
heredity alone would make them, that they have an added size due to 
a growth stimulus produced by hybridization, but that this added stimu- 
lus is largely dissipated in F2, which accordingly shows blended or inter- 
mediate heredity. It is evident that Mendelian dominance is wanting 
in these size crosses. It remains to inquire whether segregation also 
is wanting. The individual growth curves of Fi and F2 animals give no 
very clear evidence on this point. Too many environmental factors 
enter into the problem, such as time of year, food conditions, accumu- 
lation of fat in old age, and the like, which, while they do not affect 
the average growth curves already discussed, do obscure the question of 
