14 HELEN DEAN KING 



only one generation (the tenth) after the eighth did the sex ratio for 

 the inbred litters fall to 7iorm, in all other generations it was con- 

 siderably above the norm, the highest ratio (145.3 cf : 100 9) being 

 found in the litters of the eleventh generation. 



WTiile the sex ratios for the inbred litters of the eighth to the 

 twenty-fifth generations varied considerably, the variation was 

 much less after the twelfth generation than before (table 4). A 

 part of this variation was doubtless phenotypic, since seasonal 

 changes in temperature seem to alter the sex ratio in the rat 

 (King and Stotsenburg, '15), and probably also other agencies, 

 such as the age of the mother (King, '16 a), have a similar effect. 

 As all of the sex ratios were relatively high, however, the devia- 

 tions from the norm cannot be ascribed either to environment 

 or to chance, so they must have been due, in part at least, to the 

 manner in which the breeding animals were selected. 



A most striking uniformity in the sex ratios of the inbred 

 litters belonging in the eighth to the twenty-fifth generations of 

 this series is shown by the grouping of the data as made in table 6. 

 The lowest sex ratio (112.5 cf : 100 9 ) was found in the first group 

 (eighth to tenth generations) ; the highest sex ratio (130.7 cf : 100 9 ) 

 appeared in the second group (eleventh to thirteenth generations). 

 Between these extremes there was a difference of only 18 points, 

 while in the four following groups of litters the range of variation 

 in the sex ratios was less than 5 points. For the total of 824 

 inbred litters the sex ratio was 122.3 d^ : 100 9 . This latter ratio 

 was not due to an abnormal preponderance of males in a few 

 sets of records, but was based on a series of data that in seventeen 

 out of eighteen cases showed an excess of males greater than that 

 considered as normal for the species. The results obtained, 

 therefore, seem to indicate that by selecting breeding animals 

 from litters that contain an excess of males, the sex ratio can be 

 swung in the direction of the selection, although the line is contin- 

 ually inbred, brother and sister. There was in this case, however, 

 no cumulative effect of the selection. The sex ratios were more 

 uniform in the later generations than in the earlier ones, but they 

 were no higher. It is rather an odd coincidence that the sex 

 ratios in the inbred litters of the eighth and of the twenty-fourth 

 generations were exactly the same (118.2cf : 100 9). 



