SIZE. 



45 



subsequently the males are heavier. The average adult weight of a 

 female is about 400 grams, that of a male about 420 grams. 



Race B animals of both sexes weigh on the average about 80 grams at 

 birth (see fig. 1), but females grow at first a little faster than males, 

 so that between 10 and 50 days of age females are slightly heavier. 

 But the males soon catch up with the females and from 50 days on are 

 heavier. The same difference between the growth curves of the two 

 sexes is observable here, as in Cavia cutleri. The phenomenon is pos- 

 sibly a general one among mammals. Earlier maturity of the female 

 is attended by more rapid growth, but the ultimate weight attained by 

 males is greater. There is no indication in our observations that the 

 attainment of sexual maturity is followed by any slowing-up of the 

 growth rate in either sex. 



In the growth of both C. cutleri and of race B, as in other growth- 

 curves to be described, the curve is at first concave upward, but later 

 becomes convex upward. This agrees with observations on rabbits, 

 fowls, and other organisms, and its significance has been discussed 

 elsewhere (Castle et at., 1909). 



Age in Days 40 



80 



120 



160 



200 



240 



280 



320 



360 



400 



Fig. 2 — Growth curves of race B and cutleri males and of their male hybrids, both Fi and F2. 



Fj hybrid males (from the cross <? cutleri X 9 race B or C, fig. 2) 

 weigh about 85 grams at birth, i. e., they are slightly heavier than the 

 young of either pure race, a lead which they retain throughout subse- 

 quent life. At maturity they weigh about 890 grams, as compared 

 with 800 grams, the average adult weight of race B males, and 420 

 grams, the average adult weight of pure cutleri males. The females 

 (fig. 3) weigh about the same as the males at birth, or are even a little 

 heavier, but soon begin to grow less rapidly, weighing about 750 grams 

 when 1 year old. The F 2 hybrids of both sexes are smaller than the 

 Fi hybrids from birth on, a fact of undoubted significance. (See figs. 

 2 and 3.) The superior growth impetus which was produced by 



