44 



INHERITANCE IN GUINEA-PIGS. 



B hybrids, and in no case were hybrids from the two races bred with 

 each other. Nevertheless, the results obtained in the two cases were so 

 similar that for statistical purposes it was thought best to combine 

 them. Race B is taken as the standard guinea-pig race with which the 

 hybrids are compared. 



For a period of about a year and a half all cutleri individuals in the 

 laboratory, whether of pure race or hybrids, were weighed two or three 

 times a month. In this way records were obtained from which growth 

 curves, averages of weight, etc., can be deduced. The repeated and 

 frequent weighings allow the detection of periods of depression due to 

 illness or poor feeding. Due allowance has been made for all such 

 observations, as well as for increase in weight of females through 

 pregnancy. Nevertheless, observations on weight are at best not 



cf RaceS. 



200 



Age in Days 40 



FIG. 1. Growth-curves of C. cutleri and of race B guinea-pigs, the growth-curve of 

 each sex being shown separately. 



altogether satisfactory, since they are subject to fluctuation through 

 conditions of food, accumulations of fat when maturity has been 

 reached, etc. Greater value attaches to the bone measurements of 

 fully adult individuals (over 1 year old) so far as individual varia- 

 bility is concerned. But the observations on weight afford a basis 

 entirely satisfactory for the determination of average sizes and average 

 growth curves in different classes of hybrids. Incidentally they afford 

 a control on the bone measurements, for they indicate cases of abnormal 

 growth (through disease, fighting, or other cause) and allow of either 

 remedying conditions or rejecting suspicious material. 



Pure cutleri young of both sexes are of about the same average weight 

 at birth, viz, 50 grams (see fig. 1). The females at first grow a little 

 faster than the males, a fact perhaps correlated with their earlier sexual 

 maturity. At about 50 days of age the two sexes are of practically the 

 same weight, the males having again caught up with the females, and 



