206 THE ACTION OF THE LIVING CELL 



have pointed out that the popular conception that in- 

 breeding results in the degeneration of a stock is er- 

 roneous. The latter author calls attention to the fact 

 that the dire effects usually associated with inbreed- 

 ing are due to environmental rather than genetic fac- 

 tors. In other words, when animals are inbred in 

 breeding establishments, sufficient attention is not 

 given to the prerequisites for successful breeding, 

 which have been discussed in the beginning of this 

 chapter. Indeed, provided sufficient attention is paid 

 to such factors as diet, cleanliness and provision for 

 exercise, it is entirely possible to breed from poor stock 

 a strain showing a high natural resistance. In this 

 connection Greenman and Duhring remark, 'Tf for 

 any special reason it is necessary to breed from a poor 

 stock, it is by no means a hopeless task to rebuild 

 lost vigor." And again, "Brothers and sisters may be 

 mated with no unfavorable results when breeders are 

 carefully selected." This means simply that provided 

 the brothers and sisters are in a high state of natural 

 resistance we may expect their offspring to exhibit 

 a similar natural resistance. 



The conclusions of the investigators just cited are 

 in harmony with the experimental results of the 

 author. In the case of the latter, however, the in- 

 creased resistance of the breeding stock was achieved 

 by means of cytost injections rather than by the more 

 natural methods employed by Greenman and Duh- 

 ring. The fact that both methods of stock improve- 

 ment lead to the same end argues well for the validity 

 of the author's hypothesis that so-called natural re- 

 sistance, however developed, is due to an increased 

 tolerance towards cytost. 



