FERTILITY 105 



fertility is the one most likely to follow. Whether this 

 loss of fecundity in animals of consanguineous breeding 

 is to be attributed to in-breeding per se or whether it is 

 due to the rapid fixing of a tendency to sterility already 

 existing in the family, it is nevertheless true that there 

 exists a certain amount of probability that continuous 

 close-breeding will ultimately affect injuriously the 

 fertility of animals. This question is discussed at some 

 length under " In-breeding," Chapter XI. 



103. Cross-breeding and fertility. It naturally fol- 

 lows that if in-breeding is unfavorable to full fecundity, 

 cross-breeding must tend to develop this desirable quality. 

 Here again it is not easy to trace the increased fertility 

 which follows the mating of animals of diverse characters 

 to the sole act of crossing. Many of the cases of increased 

 fecundity due to crossing may be explained on the basis 

 of introducing the new quality of high fertility. If fertil- 

 ity is a dominant character transmitted in accordance 

 with the Mendelian principle of dominance, it is easy to 

 understand why the cross-bred animal may exhibit, as 

 it often does, a greater degree of fecundity than can be 

 accounted for on the assumption of blended inheritance 

 of this quality from both parents. What actually happens 

 is that one of the parents possesses the quality of fertility 

 in high degree and this becomes dominant in the offspring. 

 A certain proportion of the offspring, therefore, receive 

 in the constitution of the germ-plasm all the high fertility 

 which is an inherent part of the germ substance of the one 

 parent. It should also follow that a certain proportion of 

 the offspring inherit unchanged the tendency to low fecun- 

 dity characteristic of the other parent. It must be ad- 

 mitted that evidences of the latter are still lacking, but it 

 should be possible by experiment to determine this point. 



