No. GOS] MULTIPLE FACTORS IN MICE AND RATS 473 



offer distinct evidence of segregation. That is to say, the 

 combination of gametes formed by hybrid parents repro- 

 duce the same zygotic types as did combination of gametes 

 of the pure parent races. 



The bearing of the results on the practical breeding of 

 farm animals seems clear. If a complicated and highly 

 variable character as the hooded pattern in rats may be 

 introduced in a cross with a non-hooded form and may be 

 recovered in a large proportion of the generation, we 

 may encourage crossing as a favorable method of pro- 

 ducing new and important breeds. This will be all the 

 more apparent if we agree with the selectionists who hold 

 that the character reappearing in F2 will be at once amen- 

 able to selection and improvement in a desired direction. 

 Contamination of genes in breeding experiments which 

 are conducted on a large scale and are followed by rigid 

 selection, need not be considered as a factor of prime im- 

 portance. 



III. Physiological Factors Underlying Growth of 

 Implanted Tissue 



The study of the inheritance of spotting in mice and 

 rats has served to give a considerable amount of data more 

 or less directly comparable to that obtained in studies of 

 size inheritance; though presenting, as I shall try to show, 

 certain advantages in their freedom from environmental 

 and age influences and in their definiteness. 



"VVe may now consider a very different line of work, 

 which bears a most interesting relation to other studies of 

 the inheritance of complicated morphological and physio- 

 logical characters. The reaction of various closely inbred 

 strains of mice and their hybrids to implants of a single 

 tumor is definite, and characteristic. It nioroovcr ini- 

 doubtedly hereditary as the work of many invc-ti-ators 

 has shown. 



Tyzzer and the writer (lOK)^ havo ivp.)rttMl iv>ult^ .)!>- 

 tained in inoculating .Tai)anos(' walt/.inii- (-lu-cly in- 



bred races of common mice and their hybrids, with an 



