150 DOMESTICATED ANIMALS AND PLANTS 



pure with reference to all his characters after they have once 

 become entangled with others. 



A second method of improvement. It is very clear that here 

 we possess a means of improvement quite different from that of 

 simple selection, and, moreover, it is one that will somewhat 

 suddenly give rise to new races. The chief difficulty is to find 

 and identify the comparatively few individuals that are pure 

 with reference to all essential characters, and this is a reason 

 for reducing the characters in breeding to the fewest possible. 



As to nonessential characters the new race may remain 

 hybrid for all the breeder cares. For example, if he is trying to 

 combine amount and quality of milk, he will get along faster if 

 he pays no attention to the color of the cows, and selects only 

 the few that have the character he is after, leaving the color, for 

 the present at least, to behave as a hybrid, to be managed later 

 after the high milkers have been isolated. 



Improvement by hybridization complicated. When but one 

 nonblending character is involved, a full 50 per cent of the 

 offspring of hybrid parents is pure as to that character, it being 

 equally proportioned between the two parents. 



If, however, another character be involved, then only a small 

 proportion of the offspring that are pure as to the first char- 

 acter are pure also as to the second character, and so on for 

 additional characters. 



If all the desired characters are recessive, then all that is 

 required is to wait until the rare individual appears that has these 

 characters and no others ; but if, as in most cases, some of the 

 characters sought to be retained are dominant, the separation will 

 be a tedious operation. 



Mutation and mutants. Accidental crossing in nature is 

 constantly producing new strains, most of which go down in the 

 struggle for existence, but some of which are sufficiently vigor- 

 ous and prolific to persist. They are seldom equally vigorous or 

 equally prolific with the parent strains, else they would long ago 

 have developed into good species. These strains can of course 



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