GENETICS: W. E. CASTLE 
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an examination of the F2 growth curve, which is everywhere lower than 
the Fi growth curve. The F2 females are indeed, like Fi females, at first 
larger than young of either pure race and continue this lead for three or 
four months, but from that time on their growth rate slows up notably 
and falls below that of race B so that at maturity they are nearly inter- 
mediate in size between the parent races. The F2 females, as a group, 
have a rapid early growth like that of the Cutleri parent but lack the 
staying qualities of race B as regards growth so that from an age of about 
150 days on they fall below race B in size. Accordingly, though the 
growth curves of both parent races and of the Fi females are so far 
parallel in general course that they do not cross each other, the growth 
curve of the F2 females crosses and falls below that of race B attaining 
at length an intermediate position between Cutleri and race B. This, 
so far as heredity is concerned, we may assume to be its true position, 
WeigM 
in I 
Age in Days 40 
FIG. 2. GROWTH CURVES OF RACE B AND CUTLERI FEMALES AND OF THEIR FEMALE 
HYBRIDS BOTH Fi AND Fz. 
the added size of Fi females being due not to heredity but to a growth 
impetus arising from hybridization, in accordance with Focke's law. 
Figure 3 shows that the statements made concerning female hybrids 
are true also for male hybrids in crosses between Cutleri males and 
females of race B or race C. In this case the superior size of Fi hybrids, 
as compared with the parent races, is even more strongly in evidence, 
while F2 hybrids, though larger than either parent race for about four 
months, from that time on are of intermediate size. But for some 
reason, possibly not due to heredity, their ultimate size is nearer to that 
of race B than to that of the Cutleri parent. 
It might be supposed that the size inheritance in crosses of a wild 
species with a domesticated race may be different from that in crosses 
between different domesticated races. To test this matter crosses were 
made between females of race B and a male guinea-pig obtained in the 
