SIZE. 



47 



as season of the year, food, or the like, for it exists between lots of F t 

 and F 2 animals reared simultaneously and treated exactly alike. It is 

 clear, therefore, that a cross of distinct races, whether wild or domesti- 

 cated, brings a stimulus to growth which leads to the attainment of 

 size considerably beyond that which truly inherited size factors would 

 produce. This stimulus, however, lasts unimpaired for only a single 

 generation. But if it lasts at all into a second generation, and if its 

 persistence is not uniform in amount in all cases, it is evident that it 

 would increase the variability of F 2 as compared with F x . This is a 

 matter requiring careful consideration when the significance of increased 

 variability in F 2 is considered. 



Weight 



Age in Days 40 



Fig. 4. — Growth curves of race B males and of the male hybrids, both F! and F 2 , between the 

 Arequipa male 1002 and females of race B or similar races. 



SKELETAL MEASUREMENTS OF CAVIA CUTLERI, OF VARIOUS RACES 

 OF GUINEA-PIGS, AND OF THEIR HYBRIDS. 



It has been stated that skeletal measurements of adult animals are 

 considered more reliable criteria of size than total body-weight. For 

 this reason we have carefully preserved for study the skull and the 

 long bones of the right fore leg and right hind leg of each adult animal 

 which died a natural death or was killed, in the races whose size was 

 under investigation. Observations made by MacDowell, Detlefsen, 

 Wright, and Fish (see MacDowell, 1914, appendix) have shown the vari- 

 ous long bones of the legs to be closely correlated in length, so that it 

 seems sufficient for our purpose to measure a single one of these, and 

 we have chosen for this purpose the femur. On this we have taken the 



