GENETIC STUDIES OF RABBITS AND RATS. 7 



weight, had longer ears (10.9 centimeters), a fact which foreshadows 

 his greater adult size. He kept on growing after passing age 144 

 days and ultimately became much heavier. His bone-measurements 

 were also greater, except in one instance, femur-length, which was 

 practically the same. In the case of this pair of half-brothers, the 

 environmental conditions were substantially identical. They were 

 born within a fortnight of each other, weaned at the same age, 

 and grew up under the same seasonal conditions on similar food, yet 

 they diverged in adult weight, bone-measurements, and ear-length 

 more than the pair (2650 and 2834) which had such different environ- 

 mental conditions. 



From facts such as these, it is believed that accidental differences 

 in environment (though we have striven to avoid them) have little 

 to do with the size-differences observed among our rabbits. On the 

 other hand, it is clear that our purest races of rabbits are not perfectly 

 homogeneous genetically as regards size, since with the most carefully 

 controlled environment size-differences occur. 



Figure 2 shows the growth-curves of two female rabbits, 3099 and 

 3100, litter mates borne by the same pure Himalayan mother to the 



FIG. 2. Growth-curves of two 

 FI rabbits, litter-mates borne 

 by the same pure Himalayan 

 mother to the same pure 

 Polish sire, and kept together 

 in the same pen throughout 

 their growing-period. Note 

 difference in character of the 

 growth-curves. The rabbit 

 which was larger at weaning, 

 matures earlier and remains 

 smaller as an adult. 



10 20 Z70 300 



same pure Polish sire. Both were weaned on the same day at the 

 age 28 days. They were kept together in the same pen during 

 subsequent growth. At birth, 3099 was the larger of the two sisters 

 and maintained her superiority for about four months, when her 

 growth-rate began to slow up, while that of her sister, 3100, kept on 

 steadily. At about the age 170 days the originally smaller rabbit 

 surpassed her sister in size and held this relative position thereafter. 

 In every bone measurement she was a little larger than her sister, 

 but in ear-length they differed little, the smaller rabbit being credited 

 with having slightly longer ears. 



From the weight-records made for each rabbit it is possible to 

 construct a growth-curve, as in the cases just discussed. Such 

 curves have been plotted for each pure-bred and each FI cross-bred 



