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 128 grams ^ 



Smallest male weighed . , . . . 35 grams 



Largest female weighed in grams 



Smallest female weighed 35 grams 



The causes of this variability were in part detected. In the first 

 place it was found that the larger the Htters 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 variabiHty than the num- 

 ber of young, otherwise the heaviest individuals would occur 

 only in Htters 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 htters weigh less at birth than the members of 

 smaller htters," because they are horn 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. 



^ An exceptional case, the next highest being 113 grams. 



