FERTILITY 639 



generative tract being affected to some extent. The ovaries were 

 much overgrown by luteal tissue, and degenerate follicles which 

 are generally so common in rabbits' ovaries were relatively 

 scarce. Hammond/ however, failed to get any positive results on 

 the fertility of rabbits after treatment Vith yohimbine. We 

 obtained some evidence that yohimbine may promote mammary 

 development and the secretion of milk. * 



Effects of In-Beeeding and Ckoss-Breeding 



The fact that in-breeding may result in a _ reduced fertility has 

 been already discussed in dealing with the significance of the 

 fertilisation process (pp. 215-220).^ It was then pointed out that 

 a tendency towards sterility may be associated with a constitutional 

 loss of vigour. In the same place it was mentioned further that 

 cross fertilisation between organisms which are - allied but belong 

 to different strains often results in an increased fertility, but that 

 cross fertilisation between different species is frequently difficult 

 to accomplish while there is every gradation between a mere 

 disinclination towards gametic union and complete cross sterility. 



The differences in fertility between varieties and species when 

 crossed are discussed at some length by Darwin,^ who summarises 

 his general conclusions under seven heads. First, the laws 

 governing hybridisation in plants and animals are practically 

 identical. Secondly, there are all degrees of cross infertility. 

 " Thirdly, the degree of sterility of a first cross between two species 

 does not always run strictly parallel with that of their hybrid 

 offspring. Many cases are known of species which can be crossed 

 with ease, but yield hybrids excessively sterile ; and conversely 

 some which can be crossed with great difficulty, but produce fairly 

 fertile hybrids. This is an inexplicable fact on the view that 

 species have been specially endowed with mutual sterility in order 

 to keep them distinct." Fourthly, the degree of sterility is often 

 different in the two sorts of reciprocal crosses between the same 

 species, and hybrids produced from reciprocal crosses sometimes 

 differ in their degree of sterility. " Fifthly, the degree of sterility 

 of first crosses and of hybrids runs, to a certain extent, parallel with 

 the general or systematic affinity of the- forms which are united. 



' Hammond, loc. cit., 1921. 



^ A method of measuring the degree of in-breeding practised in any 

 particular case has been devised by Pearl. It is based on the simple and 

 definite conception that an in-bred animal has fewer different ancestors than 

 one which is not in-bred, and coefficients of in-breeding have been constructed. 

 For details of the method see Maine Agric. Exp. Station Bulletins, 215 and 243, 

 Orono, 1913 and 1915. 



^ Darwin, loc. cit. See also Origin of Species, Gth Edition, London, 1872. 



