4 GENETIC STUDIES OP RABBITS AND RATS. 



They attempt to explain the results of size-crosses with the aid of 

 genetic factors affecting the size of all parts, but relatively few in 

 number, a different potential effect on size being assigned to each of 

 the assumed factors. 



With a view to throwing further light, if possible, on this confused 

 situation, a renewed investigation of size-inheritance in rabbits 

 was undertaken by me in January, 1917. To clarify the matter, it 

 seemed desirable to obtain races as pure as possible and as unlike as 

 possible in size, to breed each separately and study its variability 

 and at the same time and under similar conditions to cross the same 

 races and study the variability of the Fi and F2 generations of off- 

 spring. For this purpose two small races of rabbits were selected, 

 Polish and Himalayan, and one large race, that of the Flemish Giant. 

 Stock was obtained from exhibitors at the last previous Boston show 

 who had been awarded prizes for the excellence of their animals in 

 these several breeds. 



In carrying out the original plan to study size-variatioji in the 

 uncrossed races, I have been only partiallysuccessful, but a good 

 series of cross-breds has been secured and studied. In the pure 

 races (plate 1) the number of adult individuals studied was, Polish 

 23, Himalayan 8, Flemish 5. Adult cross-breds have been produced 

 as follows: Polish X Flemish, Fi 27, Fj 137; Polish X Himalayan, 

 Fi 25, Fa 55 ; Himalayan X Flemish, Fi 17, Fj 70. The total number of 

 adult individuals studied for size-characters in this investigation is 

 367. The characters studied are weight, ear-length, and the fol- 

 lowing bone measurements: (1) skull length, (2) skull width anterior 

 to orbit, (3) skull width posterior to orbit, (4) length of femur, (5) 

 length of tibia, (6) length of humerus. 



Weights were taken at intervals of about two weeks, from the 

 time of weaning to the attainment of full growth. As animals are 

 apt to increase in weight through the accumulation of fat after the 

 cessation of general growth, the adult weight of an animal is taken 

 as the maximum weight attained under one year of age. Small 

 races of rabbits attain maturity earlier than large ones. For example, 

 PoUsh rabbits are usually full-grown at ten months of age, whereas 

 Flemish may grow slowly after they are one year old. The skeletal 

 measurements studied are based for the most part on animals which 

 had attained the age of fourteen months, but some measurements 

 have been used of younger animals, when it has been found by com- 

 parison of different measurements that full growth had probably 

 been attained. Adult ear-length is attained much earlier than full 

 body-size. Growth of ears is practically completed at twenty weeks 

 of age. Accordingly, it has been possible to use the ear-measurements 

 of animals which died before they attained full body-size, and so 

 the totals for ear-length studies are greater than those for weight 



