2 BULLETIN 1090, U. S. DEPARTMENT OF AGRICULTURE. 



vigor and fertility, and leading ultimately to the appearance of 

 monstrosities. On the other side, however, we find that most of 

 the modern improved breeds of livestock originated in rather close 

 inbreeding of selected stock. Systematic livestock breeding began 

 in England about the middle of the eighteenth century with the 

 attempt of Robert Bakewell to improve the native Leicester sheep 

 and Longhorn cattle. Bakewell had definite views on the character- 

 istics which he wished to combine in his animals, but he departed 

 most from the prevailing customs of his time in his use of close in- 

 breeding for the purpose of fixing these desired characteristics. His 

 methods were followed in the foundation period of most of the other 

 British breeds. In the course of time, however, certain unfortunate 

 characteristics, such as low fertility in the Duchess family of Short- 

 horns, came to be attributed to the inbreeding. At the present time 

 there is much difference of opinion among practical breeders about 

 the effects of inbreeding. 



The remarkable increase in size and vigor which often follows the 

 crossing of different varieties was noted by the early plant breeders. 

 Darwin made carefully controlled experiments on the effects of self- 

 fertilization and crossing of various plants. In general, those species 

 with mechanisms facilitating cross-fertilization suffered an obvious 

 decline in vigor when self-fertilized, while those without such a 

 mechanism suffered no ill effects. He found that little or no im- 

 provement followed crossing within a self-fertilized strain, while 

 marked improvement was the rule in crosses between such strains. 

 In applying his results to livestock breeding Darwin pointed out 

 that the advantage of close inbreeding in retaining characteristics 

 might outweigh some loss in constitutional vigor. 



Darwin's work on plants was followed by the experiments of 

 Crampe and Ritzema-Bos with rats and of Weismann and Von 

 Guaita with mice. The decrease in fertility and increase in sterility 

 noted by all of these writers and the increased susceptibility to dis- 

 ease and the appearance of abnormalities noted by Crampe have 

 long done duty as the stock examples of degeneration through in- 

 breeding. 



The problem has been attacked from a new viewpoint since the 

 rediscovery of Mendel's law. The experiments of Castle, Moenkhaus, 

 Hyde, Went worth, and others with the fruit fly, DrosopMla melano- 

 gaster, those of G. H. Shull, East, Hayes, Jones, Collins, and others 

 with maize, and those of Miss King with rats have been rapidly 

 bringing this subject into line with the current theory of heredity. 



The experiments with guinea pigs described in this bulletin have 

 given results which agree in the main with those of the authors 

 mentioned above, although appearing at first sight somewhat difficult 



