FERTILISATION 217 



Von Guaita.i and Boe,^ in describing 'the effects of in-breeding 

 in mice and rats respectively, have recorded a steady decrease of 

 fertility in successive generations. 



King,^ on the other hand, found no reduction in the fertility of 

 rats in-bred, for twenty-five successive generations, by brother and 

 sister mating.* 



Castle and his collaborators,^ as a result of an investigation upon 

 the same question in the pumice-fly {Drosophila mmpelo'phila), 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 

 experiments showed further that in-breeding results in strains of 

 unequal fertility. The less fertile were eliminated by the product- 

 iveness being differential, so that only the more fertile persisted. 

 Moreover, whereas complete sterility was marked in the first part 

 of the experiment, in the later generations it has almost completely 

 disappeared. Moenkhaus,^ and others, in similar experiments on 

 in-bre.eding Drosophila, found likewise that though sterility increased 

 in the earlier generations the later ones were fertile. 



East ' interprets these results on the hypothesis that in-breeding 

 ■ produces homozygous individuals (to use Mendelian terminology) 

 and that these when sterile are eliminated. He illustrates his 

 conclusions by reference to experiments on maize in which in- 

 breeding produces different results in different lines, showing that 

 segregation of certain factors influencing fertility has taken place. 



' Von Guaita, "Versuche mit Kreuzungen von verschiedenen Eassen der 

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



^ Bos, " Unteraucliungen ueber die Folgen der Zucht in engster Blutver- 

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



^ King, "Studies in Ih-breeding," jom: of Exp. Zool., vol. xxvi., 1918. 



^ Westermarck attributes the practice of exogamy (or marriage outside 

 the clan or family) in man to an instinctive aversion to marriage and sexual 

 intercourse between persons who have lived together closely through early 

 youth, and this mental characteristic is supposed to have arisen through natural 

 selection in view of the needs of the species which would suffer as a result of 

 in- breeding. In this theory Westermarck correlates " three parallel groups of 

 facts . . . the exogamous rules, the aversion to sexual intercourse between 

 persons living together from childhood, and the injurious consequences of 

 in-breeding " (The History of Human Marriage, 5th Edition, in three volumes, 

 London, 1921). Heape has a different theory of the origin of exogamy, 

 attributing it to the instinct which impels the errant male to seek a strange 

 female for his sexual gratification, and points out that when the pair are not 

 in accord the sexual stimulus for ovulation may not occur (Sex Antagonism, 

 London, 1913. See also Frazer, Totemism and Exogamy, London, 1911). 



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

 etc., upon the Fertility and Variability of Drosophila," Proc. Amer. Acad, of Arts 

 and Sciences, vol. xli., 1906.. 



" Moenkhaus, "The Effects of In-breeding and Selection on Fertility, 

 Vigor, and Sex Ratio on Drosophila," Jour, of Morph., vol. xxii., 1911. 



^ East and Jones, In-breeding and Out-breeding, Philadelphia and London. 

 This monograph contains a valuable discussion and numerous references to 

 literature. 



