230 GENETICS AND EUGENICS 



group were similar in constitution as regards factors affect- 

 ing fertility, and that some dissimilarity is necessary to 

 enable the pollen of one individual or variety to grow vigor- 

 ously on the stigma of another individual or variety. The 

 phenomenon of self-sterility accordingly involves the prin- 

 ciple of heterosis. (See Chapter XXVII.) 



In the great majority of animals, as in many plants, self- 

 fertilization is rendered wholly impossible by separation of 

 the sexes. The same individual does not produce both eggs 

 and sperm, but only one sort of sexual product. But among 

 sexually separate animals the same degree of inbreeding 

 varies in its effects. The closest degree, mating of brother 

 with sister, has in some cases no observable ill effects. Thus, 

 in the case of a small fly, Drosophila, my pupils and I bred 

 brother with sister for fifty-nine generations in succession 

 without obtaining a diminution in either the vigor or the 

 fecundity of the race, which could with certainty be attrib- 

 uted to that cause. A slight diminution was observed in 

 some cases, but this was wholly obviated when parents were 

 chosen from the more vigorous broods in each generation. 

 Nevertheless crossing of two inbred strains of Drosophila, 

 both of which were doing well under inbreeding, produced off- 

 spring superior in productiveness to either inbred strain. 

 Even in this case, therefore, though inbreeding is tolerated, 

 crossbreeding has advantages. 



In the case of many domesticated animals, it is the opin- 

 ion of experienced breeders, supported by such scientific 

 observations as we possess, that decidedly bad effects follow 

 continuous inbreeding. Bos (1894) practiced continuous in- 

 breeding with a family of rats for six years. No ill effects 

 were observed during the first half of the experiment, but 

 after that a rapid decline occurred in the vigor and fertility 

 of the race. The average size of litter in the first half of the 

 experiment was about 7.5, but in the last year of the experi- 

 ment it had fallen to 3.2, and many pairs were found to be 

 completely sterile. Diminution in size of body also attended 

 the inbreeding, amounting to between 8 and 20 per cent. 



