10 INHERITANCE IN RABBITS 



brother, 2 young out of a litter of 3. Both were males. The larger one, 

 although apparently healthy, failed to breed; the smaller one was not 

 tested. Accordingly, out of 5 pure-bred lop-eared rabbits with which we 

 have experimented, 2 (a male and a female) were infertile one of the 

 two largely so, and the other completely. Infertility has also been 

 encountered among a few of the female descendants of this lop-eared stock 

 produced by cross-breeding, but in no other stock of rabbits with which 

 we have experimented. Sterile individuals have not been observed among 

 half-blood lops of generation F t , but a few have occurred in later genera- 

 tions. In the majority of cases, however, the sterile individuals have been 

 three-quarter-blood lops. 



From these facts we conclude that a tendency to sterility is inherent in 

 the lop-eared stock used, and is transmitted, not to the immediate off- 

 spring (FJ if they are cross-breds, but to the next generation, when it 

 is produced by a back-cross between F 2 and the pure lop-eared stock; 

 less frequently sterility reappears in F 2 , produced by breeding the half- 

 blood lops inter se. We should expect the infertility to occur only half 

 as frequently in this latter sort of mating as in the former, where it has 

 been oftenest observed. On the whole, it seems probable that a ten- 

 dency to sterility is inherited in rabbits, as in Drosophila (see Castle et al. y 

 :o6), after the manner of a Mendelian recessive character, i. e., skipping a 

 generation in crosses. 



Why lop-eared rabbits more than other breeds should show a tendency 

 to sterility is not known; but as they are extensively inbred, it seems highly 

 probable that inbreeding is largely responsible for this sterility. The 

 lop-eared character is one which, from the manner of its inheritance, we 

 may be sure, has been built up slowly as the result of selection. In this 

 process inbreeding must have been continuously practised, for since every 

 out-cross would result in loss of half the ground gained by selection, it 

 would be practised only when absolutely necessary. 



At birth rabbits have ears quite undeveloped, and the ears do not attain 

 their full growth until an age of 5 to 8 months have been reached. Ear- 

 growth is well advanced, however, at 20 weeks, after which time it becomes 

 very slow. Accordingly 20 weeks has been found a convenient age at 

 which to institute comparisons as to ear-character between different lots 

 of rabbits. Frequently, however, it is impossible to rear an entire litter of 

 rabbits to the age of 20 weeks, in which case an earlier determination 

 of ear-character becomes desirable. For this reason, after some experi- 

 mentation, we adopted the plan of making weekly measurements of the 

 ear dimensions at ages from 2 to 20 weeks inclusive. This process, while 

 laborious, fully eliminates errors due to observation, as well as those due 

 to temporary growth conditions. 



The weekly observations upon each rabbit included taking its weight, 

 the maximum length and maximum width of its right ear, and finally the 



