ANIMAL BREEDING 205 



of mice, distinct differences were noticed. Those born 

 of immunized mothers exhibited a thick, sleek fur, in 

 marked contrast with the rougher coats of the young 

 members of the non-immunized colony. Similarly 

 the former surpassed the latter in activity and virility. 

 Indeed, in these respects the young born of the immun- 

 ized mothers approached the behavior of young wild 

 mice. This is especially interesting, since Donald- 

 son has pointed out that in the case of the albino rat, 

 the young behave similarly to wild rats if they are 

 born of hardy stock under ideal conditions. The young 

 born of the immunized mothers also gained in weight 

 more rapidly than did those of the other group. This 

 is in accord with the findings in the experiments with 

 guinea pigs referred to previously. 



Greenman and Duhring (1932) in their discussion 

 of the breeding of albino rats state that in addition to 

 the observance of the various environmental factors 

 noted in the preceding discussion, one should "select 

 animals for breeding which shall have been reared 

 for one or more generations in an exercising cage dur- 

 ing the period of rapid growth from 25 to 130 days 

 of age." This advice is based upon the empirical 

 observation that by so doing one obtains a breeding 

 stock having a higher natural resistance than is other- 

 wise the case. In view of the discussion in Chapter 

 VII, and the experiments with guinea pigs and mice, 

 it should be clear that the utility of exercise in im- 

 proving a breeding stock must depend upon the re- 

 lease of cytost by this means. This, as we have seen, 

 results in the production of anticytost, and a conse- 

 quent increase in natural resistance. 



Greenman and Duhring, as well as King (1919), 



