Experimental Study of Growth 243 
3) I, 4, 4 young, and 5, 2, 7, 6 young. Larger litters appear 
on an average during warm weather. Gestation takes from 
9 to 10 weeks, so that young born in July are influenced by 
conditions existing in May, and those in November by August 
conditions, etc. At birth male guinea pigs average 70.8 grams 
and females 70.1. The individual variation is very great. 
For instance : — 
Largest male weighed - , 7 , . 128 grams? 
Smallest male weighed . i . . 35 grams 
Largest female weighed . é j 3 - III grams 
Smallest female weighed . ‘ : F 35 grams 
The causes of this variability were in part detected. In the first 
place it was found that the larger the litters the smaller the pigs. 
“At first sight this seems easily explained as a mere ratio of food 
supply and demand; this would accord with certain views of 
Herbert Spencer, but not with the facts of nature.” There 
must be also some other cause for the variability than the num- 
ber of young, otherwise the heaviest individuals would occur 
only in litters of two or of one, but this is not the case. The 
explanation of this fact is found in the length of the gesta- 
tion period, which is shorter the larger the litter. ‘The indi- 
viduals of larger litters weigh less at birth than the members of 
smaller litters,’ because they are born sooner or, in other words, 
have not been growing so long. This is shown in the following 
table. The average time of gestation is 67 days. 
No. or Younc AVERAGE GESTATION IN Days 
I 68.7 
2 67.6 
3 66.7 
4 67 
5 
“ 66 
7 
8 
1 An exceptional case, the next highest being 113 grams. 
