244 Experimental Zoology 



The weight varies also with the season of the year, probably 

 in consequence of more favorable nutrition at certain times. 



The individuals of the same litter vary in weight, as shown in 

 the following table : — 



The young differed in weight between 



o- 3 grams in 12 cases 



4- 7 grams in 19 cases 



8-1 1 grams in 10 cases 



12-15 grams in 6 cases 



16-19 grams in 6 cases 



20-23 grams in 4 cases 



23-27 grams in i case 



35 grams in i case 



This difference in weight between members of the same litter is 

 probably due to differences in position of the embryo in the uterus, 

 or to the number of young in each uterus, three young being in 

 one uterus and only one in the other. If this is true, odd num- 

 bers should show greater differences, and this is often the case. 

 For a few days after birth male guinea pigs lose weight; 

 jemales gain weight. The females average, when born, a little 

 less than the males, but they gain during the first days, so that 

 they may actually then weigh more, and this "advantage is long 

 maintained." A more minute examination shows that both sexes 

 really lose after birth, but the males more ; the females begin very 

 quickly to gain, and obscure the initial loss. In both males and 

 females the growth during the first few days is less than that from 

 the fifth day onward. It is not until the twenty-ninth day that 

 the male catches up, weighing 203 grams as against 203.7 for 

 the female. When the adult condition is reached, the males 

 weigh much less than the females. After the first month to the 

 end of the first year the males average more. During the second 

 year the averages are too variable for generahzation, although 

 the males weigh somewhat less on the average. During the 

 youth of the guinea pigs one point comes out clearly, viz. that 

 ''each individual appears to be striving to reach a particular size." 

 Thus if an individual grows for a period excessively fast, there 



