EXPERIMENTAL EVOLUTION. 125 



Thus Japanese waltzing females crossed with non-waltzing males give 

 a sex-ratio of 44.0 =±= 7.4, while Japanese w^altzing males crossed with 

 non-waltzing females give a sex-ratio of 118.2=^3.8. The latter result 

 is commonly obtained in hybrid combinations, but the former repre- 

 sents a departure from the normal type requuing explanation. 



Dr. Little advances the following h>T)othesis as that which best fits 

 all of the observed experimental facts. In some of the females of the 

 inbred Japanese waltzing race there is a recessive lethal factor which is 

 sex-Unked. Such females would transmit the lethal factor to one-half 

 their male progeny. Males of this sort, having the lethal in an unbal- 

 anced condition, would not survive, thus producing a sex-ratio of 1 

 male to 2 females or 50.0 in the progeny of such females. All surviving 

 males would, by hypothesis, lack the lethal, and therefore could not 

 transmit any peculiarity of the sex-ratio to their progeny. The result 

 of the cross (Japanese waltzing male by non-waltzing female) is thus 

 explained. One-half of the female progeny of the lethal-bearing females 

 should theoretically be homozygous normals and one-half should trans- 

 mit the lethal. In more advanced hybrid generations descended 

 through Japanese waltzing females an excess of females should thus be 

 produced. The exact ratio would depend upon the proportion of lethal- 

 bearing to homozygous normal females in the population. Actually 

 such an excess of females has been obtained. The sex-ratio of ad- 

 vanced hybrid progeny descended through the Japanese waltzing 

 female line is 78.7 ±2.9. 



The size of litter also affords supporting evidence for the presence 

 of a lethal. Frequency polygons for litter-size in Japanese waltzing 

 and in non-waltzing females have been compared by a %' test. The 

 58 litters from Japanese waltzing females average 3.38 young per litter, 

 while those from non-waltzing females (100) average 5.93. The odds 

 are greater than 1 in 100,000 against the distribution curves being the 

 same. Since Japanese waltzing females should be of two general types, 

 (a) those producing small litters, due to the lethal, and (6) those pro- 

 ducing larger litters when free from the lethals, the result falls in line 

 with the sex-ratio evidence. 



It seems clear, therefore, that certain females of the closely inbred 

 Japanese waltzing-mouse race are transmitting a recessive, sex-linked, 

 lethal factor. This is, it is believed, the first case of a sex-linked lethal 

 in mammals and the first case of sex-linkage in rodents. 



DIFFERENCES IN RESISTANCE OF SPERM OF DIFFERENT SPECIES 

 TO ACID SOLUTIONS OF VARIOUS STRENGTHS, IN RELATION TO THE 

 SEX-RATIO. 



In the annual report of the Institution for 1919 the hypothesis was 

 stated that certain peculiarities in the sex-ratio might be due to varia- 

 tions in the quality of the internal secretions of the female reproductive 



