46 



INHERITANCE IN GUINEA-PIGS. 



hybridization has not been retained in the second-generation offspring, 

 which sink as regards weight to a position intermediate between the 

 parent races. Nevertheless the F 2 hybrids are nearer to race B than 

 to cutleri in adult size, which fact suggests that not all the growth 

 impetus furnished by hybridization has yet been dissipated. In form 

 of growth curve the F 2 hybrids are also intermediate. The growth 

 curve at first rises rapidly, due in part perhaps to the good milk-giving 

 qualities of then* vigorous F! hybrid mothers, but in part probably to 

 inheritance of cutleri qualities, since the cutleri growth curve is a rela- 

 tively steep but low one, indicating rapid growth at first and early 

 maturity. The F 2 hybrids also grow rapidly at first, being consider- 

 ably heavier than race B animals until an age of 120 to 150 days has 

 been reached. Then they fall below and stay below the weight of 

 race B animals, running a course nearly parallel with that of pure 

 cutleri animals, whereas the growth curve of the F! animals more nearly 

 approached that of race B animals. 



FIG. 3. Growth curves of race B and cutleri females and of their female hybrids, both F! and Fj. 



While we are on this subject it may be well to refer to the growth 

 curves observed in the cross between the Arequipa male, 1002, and 

 females of race B (or of similar character). (See fig. 4.) The data for 

 the growth curves of males are more complete in this case than the 

 data for females and accordingly only the former will be considered. 

 The F! animals are of great size and vigor, attaining an average adult 

 weight of over 1,200 grams. The F 2 animals are even larger at birth 

 than the F 1 animals, a fact which indicates that the size of the mother 

 has something to do, other than through heredity, with the size of the 

 young at birth, for the F 2 young rapidly lose the lead which they had 

 in weight at birth over their F! parents, and subsequent to 40 days of 

 age fall below them in weight. At maturity they weigh less than 1,000 

 grams, having lost more than half of the gain which the F! animals 

 showed over race B animals. This difference, it should be stated 

 emphatically, is not due to environmental conditions of any sort, such 



