FERTILITY IN ANIMALS 559 



generation of germ cells begins in synapsis. They conclude, therefore, 

 that sterility in pheasant hybrids depends upon the inability of homolo- 

 gous chromosomes derived from different species to conjugate normally. 

 In the mule, which apparently receives a different number of chromo- 

 somes from each parent, a morphological cause for such a difficulty 

 obviously exists, but fundamentally the difficulty must be physiological, 

 for it exists as strikingly in hybrids between species having the same 

 numbers of chromosomes as in the rarer cases where the species have 

 different chromosome numbers. We have already discussed this prob- 

 lem at length. 



Fertility as Related to Heterozygosis In another place we have 

 discussed the hypothesis that heterozygosis in and of itself has a favorable 

 effect upon vigor and fertility. This hypothesis is difficult to prove or 

 to disprove. With the facts, however, there can be no question. Cross- 

 breeding definitely does in specific cases lead to an increase in vigor and 

 fertility, a fact which has long been known. But it appears more prob- 

 able, as Jones has shown, that this increase is due to the establishment 

 of a more excellent factor-complex than to any mysterious stimulation 

 effect of the heterozygous condition. At the same time the possibility 

 of an enlarged expression of the heterozygous condition of a given pair 

 of allelomorphs must not be denied, but like other effects of heterozy- 

 gosis, it is probably a condition depending upon the specific nature of 

 the factors concerned. As a generalization, however, it must be taken 

 as not proven; certainly the work with Drosophila, which is based upon 

 more definite knowledge of the Mendelian factors than any other investi- 

 gations to which we can refer, does not provide evidence in support of it. 

 The solution of the problem has in it much of practical importance, for 

 upon the hypothesis of heterozygosis it should be impossible to build up 

 a breed which would reproduce in full the complete set of excellent char- 

 acters of the cross-bred. If, however, a more favorable combination of 

 factors is responsible for the excellence of cross-bred animals, then it 

 should be possible by careful breeding to fix them in a new breed. 



Fecundity in Fowls. It is a genuine pleasure in a mass of contradic- 

 tory and illy digested data to meet with something which gives hope for 

 the same definiteness with regard to the problem of the inheritance of 

 fecundity that has been attained in the analysis of the inheritance of 

 other more clearly defined characters. We cannot, therefore, but com- 

 mend the patient investigation and brilliant analysis to which Pearl has 

 subjected the problem of the inheritance of fecundity in the domestic 

 fowl. Many criticisms have been launched against his conclusions, it 

 is true, but it is highly probable that these criticisms involve a funda- 

 mental misconception of the nature and results of scientific knowledge. 



Pearl's results deal particularly with winter egg production in the 



