160 



INBEEEDING AND OUTBREEDING 



guinea-pig and the rabbit. One need go no further than 

 to cite the work of Castle and his students at the Bussey 

 Institution of Harvard University, the work of Miss King 

 at the Wistar Institute of Anatomy and that of Wright 

 at the Bureau of Animal Industry of the United States 

 Department of Agriculture. The painstaking researches 

 of these investigators show without question that the 

 effect of crossing on animals is the same as upon plants. 



Age in Days 40 



160.- 



FiQ. 35. — Growth curves of males of race B guinea-pigs and Cavia cutleri and their Fi 

 and Fi hybrids. (After Castle.) 



The results from one genus is typical of them all. 

 Castle ^^ made a cross between a domestic guinea-pig and 

 a wild cavy, Cavia cutleri. The first generation hybrid 

 males weighed about 85 grams at birth, which is slightly 

 more than the young of either pure race, and retained this 

 lead throughout their subsequent life as is shown by the 

 growth curve in Fig. 35. At maturity they weighed about 

 890 grams, as compared with 800 grams for the guinea- 

 pig ancestor and 420 grams for the cutleri ancestor. 



The second generation hybrids of both sexes were 



