EFFECTS OF INBREEDING ON THE SEX RATIO 6 



would be expected to be more potent than any other kind in 

 producing an alteration in the sex ratio. In these experiments, 

 therefore, brother and sister matings only were used to obtain 

 strictly inbred litters from which all females used for breeding 

 were taken. The plan of breeding that was followed through the 

 first twenty-five generations of these animals was this: Females 

 A and B, as well as all of the females in their respective lines 

 that were subsequently used for breeding, were paired twice with 

 a litter brother and then twice with an unrelated male taken 

 from the stock colony. Sex records for the first two litters 

 produced by any group of females might be expected to show 

 whether inbreeding had any effect on the sex ratio; sex data for 

 the third and for the fourth litters cast by these same females 

 would, it was hoped, indicate whether the male or the female 

 was responsible for the alteration, if any, in the sex ratio. For 

 convenience the litters obtained from the mating of inbred fe- 

 males with stock males are here designated as 'half-inbred' 

 litters; no animals from such litters have ever been reared. 



Emphasis should be placed on the fact that, with few exceptions, 

 the sex data given in this paper were obtained by examining the 

 litters very soon after their birth. The sexes can readily be 

 distinguished at this time, as Jackson ('12) has shown, and if 

 accurate sex data are wanted it is imperative that they be taken 

 as soon as possible, since the young that are stillborn, or those 

 that die soon after birth, are usually eaten by the mother within 

 a few hours. 



In order to keep track of a large series of animals it was neces- 

 sary to find some way in which the pedigree of any particular 

 individual could be told by a glance at the record card. The 

 scheme of marking devised, which is outlined below, has proved 

 to be very convenient and also most satisfactory for the filing 

 of permanent records. The letter A or B is used to show from 

 which of the two females, A or B, the animal was descended, and 

 thus places the individual in its proper series. The serial letter 

 is preceded in all cases by a number which signifies the generation 

 to which the animal belonged. An index number, 2, 3, or 4, 

 following the serial letter shows in which of the mother's litters 



