EFFECTS OF INBREEDING ON FERTILITY AND VIGOR 357 



months with males that were known to be fertile. A postmortem 

 examination of the reproductive organs from several of these 

 semi-sterile females showed, in every instance, an inflamed con- 

 dition of the ovaries or of the uterus which would render repro- 

 duction impossible. Barrenness in these cases was doubtless due 

 to disease and not to any inherent tendency to sterility. A 

 similar diseased condition of the reproductive organs has been 

 found to be responsible for the partial sterility of stock Albinos. 



2. THE CONSTITUTIONAL VIGOR OF INBRED RATS 



The best criterion by which to gauge the so-called 'constitu- 

 tional vigor' of any animal is undoubtedly its power of repro- 

 duction, since that is of the utmost importance for the continua- 

 tion of the race. There are, however, other important tests for 

 vigor that can be applied, such as the rate and extent of growth, 

 agility, mental alertness, resistance to disease, and ability to live 

 to an advanced age. According to Darwin ('78), ''the effects 

 of close interbreeding in animals, judging from plants, would be 

 deterioration in general vigor, including fertility, with no neces- 

 sary loss of excellence of form." This would seem to indicate 

 that, whatever tests were applied, closely inbred animals and 

 plants would show marked inferiority when compared with 

 individuals of the same species that were not inbred. That such 

 a sweeping generalization is not justified is shown by the results 

 of a number of recent inbreeding experiments : the work of Shamel 

 ('05) on tobacco, of Stout ('16) on chicory, and of Hayes and 

 Jones ('17) on tomatoes give no indication that self-fertilization 

 in these plants causes a loss either of vegetative or of reproductive 

 vigor; Gentry's ('05) experiments on swine, and those of Castle 

 et al. ('06) and of Moenkhaus ('11) on Drosophila show that 

 any loss of vigor that might come from inbreeding can be entirely 

 overcome by the proper selection of breeding stock. 



The present series of experiments on the rat are the first 

 recorded for any mammal in which brother and sister matings 

 were made continuously for twenty-five successive generations. 

 In the inbreeding experiments with rodents made by Crampe, 

 by Ritzema-Bos, and by von Guaita, matings were made between 



