CHAPTER IX 



STERILITY AND ITS RELATION TO INBREEDING 



AND CROSS-BREEDING 



PROBABLY the most noticeable effect of inbreeding in 

 both animals and plants is a reduction in fertility in the 

 earlier inbred generations. The experiments of Ritzema- 

 Bos 184 with rats, of Weismann 86 with mice and of 

 Wright 225 with guinea-pigs are all thus characterized. 



Miss King, 119 on the other hand, has inbred albino 

 rats for twenty-five successive generations by brother and 

 sister mating without any appreciable reduction in fer- 

 tility. Similarly, Castle 21 and his students maintained 

 the fertility of Drosophila for fifty-nine generations of 

 brother and sister mating by breeding from the most fer- 

 tile flies. ^Various lines were isolated, nevertheless, which 

 differed in the number of offspring produced, and in the 

 first part of the experiment many individuals appeared 

 which were absolutely sterile. The production of such 

 non-fertile flies became less in the latter part of the ex- 

 periment, and the average fertility of the remaining stock 

 was improved by this elimination. 



In maize the results of inbreeding are generally quite 

 serious as regards fertility. In the first place the consis- 

 tent reduction in size and constitutional vigor of the 

 plants necessitates a much smaller production of pollen 

 and ovules. The tassels are reduced in size and have 

 fewer branches. The ears are smaller and shorter and 

 oftentimes imperfectly covered with seeds even when 

 abundant pollen is available. In some cases the leaves 



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