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 
8-11 grams in Io 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 1 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 
maintainec.” 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 generalization, 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 
