FERTILITY 641 



they were Mendelian units. In the light of this conception the 

 effects both of in-breeding and of cross-breeding would seem to 

 acquire a new signification. It has been established that in some 

 insects (Drosophila) complete sterility may be a sex-linked character, 

 and the same is apparently true of the male tortoiseshell cat. It is 

 suggested that in the famous Duchess family of Shorthorn cattle 

 bred by Bates and which eventually became extinct, there was a 

 factor for sterility present, and this as a result of close in-breeding 

 kept on reappearing with increased frequency until no more fertile 

 individuals were produced. It has been shown that with Drosopkila, 

 and also with the guinea-pig, a high degree of fertility can be 

 maintained in successive generations of in-breeding by selecting from 

 the more fertile individuals. Further, there is direct cytological 

 evidence that in Drosophila sterility is correlated with the absence of 

 a particular chromosome. 1 



And conversely the increased vigour and fertility apparently 

 brought about by cross-breeding and heterozygosis may be "due to 

 the establishment of a more excellent factor-complex rather than 

 to any mysterious stimulation effect of the heterozygous condition." 2 

 At the same time it is possible that the reproductive cells even in 

 their hereditary composition are capable of being affected by their 

 environment, and it is reasonable to suppose that in the unicellular 

 organisms at any rate new and more favourable surroundings, acting 

 in conjunction with an appropriate factor-complex, may have a stimu- 

 lating and favourable influence on fertility. 



The sterility of hybrids is a very common phenomenon and has 

 received a cytological interpretation. Such infertility is, however, 

 by no means invariable. 



Wallace has cited several cases in which it has been shown that 

 hybrids between distinct species are fertile inter se. Such cases are 

 the hybrids between the domestic and Chinese geese, and the various 

 hybrids between the different species of the genus Canis. A ease of 

 a fertile hybrid between a lion and a jaguar has been recorded also. 

 The various members of the family Bovidse are known to hybridise 

 and to produce fertile offspring even though the parents belong to 

 what are ordinarily regarded as different genera. The cow (Bos taurus) 

 gives fertile male and female hybrids when crossed with the zebu 

 (Bos indicus). With the yak (Bibos yrunnicus), the gaur (Bibos gaurus), 

 the gayal (Bibos frontalis) and the bison (Bison americanns) the cow 

 has fertile female hybrids but sterile males. 3 The hybrids between 



1 For an admirable discussion of the subject see Babcock and Clausen, 

 Genetics in Relation to Agriculture, New York, 1918. This work contains many 

 references. See also Morgan, Heredity and Sex, New York, 1913. 



* Babcock and Clausen, loc. cit. 



3 Babcock and Clausen, loc. cit. 



