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254 GENETICS: W. E. CASTLE 
lives after which increase in weight slows up gradually and finally ceases 
altogether at the age of about one year. Both sexes are of about the 
same size at birth but females grow at first faster than males, attaining 
sexual maturity some weeks earlier. At about fifty days of age the sexes 
are again of the same average size, but from this time on the males are 
heavier. The general form of the growth curve in both sexes is, as in 
animals generally, at first concave upward but later becomes convex 
upward as the growth rate declines. Growth is completed at about 
one year, after which time no change in weight occurs except as caused 
by health or food conditions, or pregnancy of the females. The growth 
curve of C. Cutieri may be characterized as a sharply rising, flat curve. 
Compared with this the growth curve of the domesticated race B rises 
less rapidly at first but continues its rise longer. The same difference 
in the growth rate of the sexes is observed in race B as in the wild C. 
Cutieri. Females grow faster up to 40 or 50 days of age, after which 
time males permanently take the lead. The adult weight of race B 
animals is approxmately double that of Cutieri individuals. 
In figure 2 are reproduced for comparison the growth curves of the 
females of both race B and Cutieri and along with these are plotted the 
grov/th curves of Fi and of F2 females produced by crossing Cutieri males 
with females of race B and race C. These curves show that Fi females 
are larger at birth than females of either parent race and that they remain 
continuously larger having at maturity a size slightly larger than that 
of either parent race. That this increased size is not due to heredity, 
but to a growth stimulus produced by the crossing of unrelated parent 
races, a 'law of hybridization' formulated by Focke (1881), is shown by 
