208 THE PHYSIOLOGY OF REPRODUCTION 



tion frequently occurs, but that the eggs in this species also are 

 most usually fertilised by spermatozoa from another individual. 1 



It is well known that the fertility of animals which are much 

 in-bred is often reduced, but this is by no means invariably the 

 case. 2 Thoroughbred horses are notoriously in-bred, and it is 

 interesting to note that one of the earlier reports of the Royal 

 Commission on Horse-breeding states that no less that forty per 

 cent, of the thoroughbred mares in this country fail to have foals 

 each year. This relatively large amount of sterility is probably 

 due to a variety of causes, and not entirely to the results of 

 in-breeding. 



Low 3 has recorded an experiment on the effect of in-breeding 

 in fox-hounds. The particular strain is described as having 

 perished completely. Low states also that similar experiments 

 have been performed upon pigs, and, as a consequence, the 

 litters became diminished in size and frequency, while difficulty 

 was often experienced in rearing those which were produced. 



Von Guaita, 4 and Bos, 5 in describing the effects of in- 

 breeding in mice and rats respectively, have recorded a steady 

 decrease of fertility in successive generations. 



Castle and his collaborators, 6 as a result of an investigation 

 upon the same question in the pumice-fly (Drosophila am- 

 pelophila), have come to the conclusion that in-breeding tends 

 to reduce the fertility to a slight extent, whereas cross-breeding 

 has a contrary effect. 



The diminished fertility of in-bred animals may be due 

 partly to a decrease in the supply of mature ova correlated with 



1 Morgan, "Self-Fertilisation induced by Artificial Means," Jour, of 

 Exper. ZooL, vol. i., 1904. " Some Further Experiments on Self -Fertilisation 

 in dona," Btol. Bull., vol. viii., 1905. 



2 The results of in-breeding are discussed at some length by Darwin, 

 Variation of Animals and Plants, vol. ii., Popular Edition, London, 1905. 

 For a recent review of the subject see Morgan, Experimental Zoology, 

 New York, 1907. 



3 Low, The Domesticated Animals of Great Britain, London, 1845. 



4 Von Guaita, " Versuche mit Kreuzungen von verschiedenen Rassen der 

 Hausmaus," Ber. d. Naturf. Gesell., Freiburg, vol. x., 1898. 



5 Bos, " Untersuchungen ueber die Folgen der Zucht in engster Blutver- 

 wandtschaft," Biol. CentralbL, vol. xiv., 1894. 



6 Castle, Carpenter, Clark, Mast, and Barrows, " The Effects of In-breeding, 

 &c., upon the Fertility and Variability of Dros&phila" Proc. Amer. Acad. of 

 Arts and Sciences, vol. xli., 1906. 



