DARWIN 

 mon. Those individuals of their progeny which show 

 this same trait are again selected and bred together, 

 and so on until the desired result is reached. This 

 method is further helped by continued change of diet, 

 climate, or other factors of the environment. What- 

 ever means are adopted, one practice must never be 

 departed from; breeding outside the strain must be 

 rigidly prevented or the organism reverts back to the 

 original stock. To make our ideas more precise let us 

 follow Darwin and consider a particular case in some 

 detail. After deliberation, he selected the pigeon as 

 the best example of directed breeding. He kept every 

 breed he could purchase or obtain; he corresponded 

 with pigeon fanciers; he found that records of breed- 

 ing extended back into antiquity, and that the diver- 

 sity of breeds is something astonishing.^^ In addition 

 to all these reasons for his choice, there is the re- 

 markable advantage that : "Great as are the differences 

 between the breeds of the pigeon, I am fully con- 

 vinced that the common opinion of naturalists is cor- 

 rect, namely, that all are descended from the rock 

 pigeon {Columba Livia)^ including under this term 

 several geographical races or sub-species, which dif- 

 fer from each other in the most trifling respects. "^^ He 

 then tells us with the utmost care how man was able 

 to produce such astonishing variations from a single 

 parent stock: "Man can hardly select, or only with 

 much difficulty, any deviation of structure, excepting 



1'^ Origin of Species, vol. I, p. 23. 

 18 Ibid., vol. I, p. 26. 



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