6 



GENETIC STUDIES OF RABBITS AND RATS. 



mother alone, receiving all her milk until he was weaned at 71 days 

 old, when he weighed 1,530 grams, more than 2.5 times as much as 

 his half-brother at the same age. Notwithstanding this handicap 

 in his favor, he was overtaken in size by his early-weaned half- 

 brother at age 194 days, and was surpassed by him in adult weight 

 by about 150 grams. The bone dimensions of the two rabbits were 

 very similar, with a slight but probably not significant superiority 

 in favor of the late-weaned individual. In the case of neither rabbit 

 does the growth-curve show any interruption of healthy growth. 

 The form of the growth-curve is affected by the superior nutrition of 

 2834 during his first 2 months, but the ultimate size attained is not 

 influenced thereby. 



Figure 1 also shows the growth-curve of a third rabbit (2578), 

 half-brother to each of the others (2650 and 2834). He was born 



30 



25 



^ tJJ 



UJ 



^ 15 



$ 10 



5 ■ \—^ 



30 



60 90 120 150 180 210 240 270 



AGE. IN DAYS 



300 330 360 390 420 A50 



FiQ. 1. — Growth-curves in weight of three Fi rabbits which were half-brothers, all having the 

 same pure Flemish sire but each having a different pure Polish mother. Male 2578 

 was born July 26, 1917; male 2650 was born August 9, 1917; male 2834 was born Decem- 

 ber 7, 1917. Note that the rabbit which was largest as an adult (2650) was smallest 

 up to 150 days of age,- whereas the rabbit (2834) which was much the heaviest as a 

 young rabbit because he received all the milk of a foster-mother up to weaning at 71 

 days of age, nevertheless fell below 2650 in adult weight. 



just two weeks earlier than 2650, was weaned at the same age (29 

 days), but was slightly heavier at weaning and held the lead up to 

 age 144 days, when he was practically full-grown. His ears were 

 then of maximum length, 10.4 centimeters. On the same day 

 (December 17) the younger rabbit (2650), although lighter in 



