FERTILITY OF PARENT SPECIES AND HYBRIDS. 103 



zygotes may have been formed less frequently, or may have been elimi- 

 nated at an early stage. 



The only similar case in mammalian crosses which I have been able to 

 find is that described by Boyd (1914), in which the bison and domestic 

 cattle were crossed. Boyd found that his hybrids gave 60 females to 

 17 males, or a ratio of 28.33 males to 100 females. Boyd likewise 

 found sterility common in the males, similar to that in my hybrids. 



22. SUMMARY AND GENERAL CONCLUSIONS. 



(1) Crosses between C. rufescens males and C. porcellus females gave 

 completely sterile male hybrids and fertile female hybrids. By cross- 

 ing the female hybrids back to guinea-pig males, I wild hybrids were 

 obtained, which were again sterile males and fertile females. A few 

 males of this second hybrid generation, however, showed some degen- 

 erate non-motile sperm. By repeated back-crosses of female hybrids 

 to guinea-pigs, increasing signs of fertility appeared. Fertility seemed 

 to act like a veiy complex recessive character; for the results obtained 

 were what one would expect if a number of dominant factors for 

 sterility were involved, the elimination of which would give a recessive 

 fertile type. There was an enormous range of forms between hybrids 

 with no sperm and fertile hybrids with many motile sperm. 



(2) The results indicated that a completely fertile hybrid male could 

 be bred to female hybrids or to guinea-pigs, giving about the same 

 results as a normal guinea-pig male in such matings. 



(3) The secondary sexual characters of all male hybrids were normally 

 developed. 



(4) The mid C. rufescens has a smaller litter average than the guinea- 

 pig. When the wild males were bred to guinea-pig females, the size 

 of the litters was that of the guinea-pig. The female hybrids produced 

 by this cross, however, gave a litter average intermediate between that 

 of the wild and tame. By repeatedly crossing the hybrid females of 

 one generation back to guinea-pig males to produce the next hybrid 

 generation, the litter average was raised almost to that of the guinea- 

 pig itself. This is all the more interesting since guinea-pig 7nales were 

 used to raise the litter average. 



(5) Two female hybrids showed some male secondary sexual char- 

 acters. One of these with marked male instincts had abnormal ovaries. 

 Abnormal ovaries were common in the female hybrids. 



(6) The sex ratio in the hybrids showed a marked preponderance of 

 females, expecially in the early hybrid generations, i. e., in those genera- 

 tions which must have been most hybrid in constitution. 



