608 PRACTICAL PROBLEMS 



quicker, cheaper, or more thorough way of becoming acquainted 

 with a breed than through its grades. 



Disadvantage of grading. The only disadvantage that can be 

 mentioned is this, that the first results are so eminently satis- 

 factory that some promising grade is likely to be selected as a 

 sire, regardless of the law of ancestral heredity, whereupon all 

 further improvement stops. This is so likely to be the case that 

 it may be said in general that the very success of grading is the 

 greatest guaranty of its failure. 



SECTION III CROSSING OR HYBRIDIZING 



Almost the exact opposite of grading, crossing combines 

 ancestral lines of two distinct races, breeds, or varieties, in the 

 hope either of securing a blend or else of getting a fortuitous 

 combination of characters. 



This form of breeding is adapted only to the production of 

 new strains, in which it excels. Of course it so mixes blood lines 

 as to effectually destroy the influence of the ancestry and all 

 meaning and value of pedigree. Its hope is in starting a new 

 strain, which may perchance breed pure. 



The operation of Mendel's law teaches how small is this 

 chance. If this law always held with all races and characters, 

 it would of course be impossible to secure permanent strains 

 by crossing, but the fact remains that permanent hybrids have 

 frequently been secured by this method, especially among plants, 

 which is a noteworthy fact in breeding. 



Advantages of crossing. Notwithstanding the operation of 

 Mendel's law as a general principle, crossing is a fruitful source 

 of new strains. Hybridization is better adapted to plants than 

 to animals because of the need of vigorous selection afterward 

 and, therefore, of relatively large numbers. It was a favorite 

 method of plant improvement twenty years ago, but it has fallen 

 largely into disuse because of the inconstancy of Mendel's 

 middle term (the 50 per cent apparent hybrids) and because as 

 good or better results can often be secured by selection alone, 

 without destruction of the pedigree and the influence of the 

 ancestry. 





