2 HELEN DEAN KING 



has no pronounced tendency to produce an excess of male young, 

 although he found a high sex ratio (110. 9cf: 100 9) among 135 

 mice that were the offspring of brother and sister matings. The 

 question as to whether inbreeding does or does not alter the sex 

 ratio was not satisfactorily answered by any of these experi- 

 ments, for in each case the number of animals used was small, 

 and there was, apparently, no selection of the best stock for breed- 

 ing or any way of checking the results. Moreover, none of these 

 investigations were continued long enough to give evidence that 

 could be considered as conclusive. 



The effects of inbreeding on the sex ratio seemed to me to be a 

 problem of sufficient importance to warrant a careful and pro- 

 longed investigation. For if it were possible to swing the sex 

 ratio of any animal in a definite direction by factors that could be 

 controlled, one might hope to gain valuable information regard- 

 ing the nature of sex — a problem that has been a favorite subject 

 of speculation for many centuries and one that modern methods of 

 research have not, as yet, satisfactorily solved. 



1. MATERIAL, METHOD, AND SCOPE OF THE INVESTIGATION 



The albino rat (Mus norvegicus albinus) was the animal used 

 in this investigation, which was begun in 1909. Details regarding 

 the manner in which the experiments were conducted were given 

 in the first paper of this series (King, '18), but it has seemed ad- 

 visable to repeat them here in order to give a clear understanding 

 of the way in which the problem has been approached. 



The basis of the inbred strain was a litter of four albino rats, 

 two males and two females, taken from the general colony of 

 these animals maintained at The Wistar Institute of Anatomy 

 and Biology in Philadelphia. The litter was selected for the 

 purpose in view solely because of its size, not because of the 

 ancestry or the vigor of the animals. One of the two females 

 in the litter was called 'A', and her descendants form the A 

 series of inbreds; the other female was called 'B', and her de- 

 scendants are the B series of inbreds. 



Since the mating of brother and sister from the same litter is 

 the closest form of inbreeding possible in mammals, such matings 



