FERTILITY OF PARENT SPECIES AND HYBRIDS. 101 



The results therefore show that in mating- the wild C. rufescens to 

 guinea-pigs, the litter average of the Fi hybrids was about intermediate, 

 and continued back-crosses raised this average gradually. 



It may be added that the proportion of females producing some 

 fertile males or males with all evidences of fertility gradually increased 

 in each generation. Certain females in the later generations produced 

 only fertile males, but the number of young from one female was 

 necessarily small and we can not be sure but that they would have 

 given sterile sons had larger numbers been possible. However, one 

 should eventually be able to produce female hybrids with the fecundity 

 of the guinea-pig species and having only fertile sons. Combining these 

 characters with wild characters, such as the peculiar wild agouti, 

 should also be possible. 



Two abnormal females ( 9 263 i wild and 9 393 -^ wild) should be 

 recorded. The former, 9 263, had an enlarged clitoris, resembUng a 

 penis, but also all the female characters, bore 2 young, and gave milk. 

 It was difficult to keep her with a male, for as she grew older they 

 fought continually. The latter, 9 393, also had an enlarged clitoris, 

 which was very nearly of the same form and size as a normal penis. 

 The female external characters were all normal. She had no young to 

 my knowledge, but upon one occasion she showed large and abrupt 

 loss of weight, and gave milk at the same time. It is barely possible 

 that she had aborted. When kept alone for some time, and subse- 

 quently placed with a male, she allowed the male to attempt copulation. 

 "^Tien placed with females she always attempted copulation, making 

 the same sounds and going through the movements of a normal male. 

 If she was penned with a male and females, she and the male fought 

 continually for the possession of the females. She was killed at the 

 age of 2 years, and the ovaries were examined. They were abnormally 

 large, measuring about 1^ inches in length and an inch in width. The 

 foUicles were greatly distended, some measuring 0.75 inch in diameter. 



Abnormal ovaries of this type were not uncommon in other female 

 hybrids which bore young and were otherwise perfectly normal in all 

 respects. The viscera of many female hybrids were examined, but no 

 data were taken on the occurrence of this type of abnormality. 



21. THE SEX RATIO IN THE HYBRIDS. 



The many recent experiments with sex-linked and sex-repelled char- 

 acters have led to the current opinion that sex itself is a Mendelian 

 character, and that one sex is homozygous while the other is hetero- 

 zygous for sex-determining factors. One would expect an equality of 

 the two sexes in the long run on this hypothesis; but when an excess 

 of one sex occurs consistently, it is supposed that the heterozygous 

 parent fails to produce the two kinds of gametes in equal numbers, or 



