CHAPTER XXXIII 

 ANIMAL BREEDING 



Reproduction in Higher Animals. — Whereas the 

 plant breeder has a considerable range of reproductive 

 processes at his disposal and can often choose the one 

 best suited to his purposes, the animal breeder is limited 

 to a single one. There are two sexes in all higher animals, 

 so that every individual has two parents and is therefore 

 likely to be hybrid for a number of genes and their cor- 

 responding characters. If the breeder makes a cross be- 

 tween the purest of races he cannot expect the hybrids 

 to be all alike, as he could if dealing with self-pollinated 

 plants, because the parents are probably never exactly 

 alike in all their genes. 



On Interbreeding. — Since there are always two sexes 

 the breeder of higher animals cannot self-fertilize his 

 first generation hybrids but must mate them with one 

 another. In some species of animals it is possible to 

 mate brother and sister without ill effects, while in others 

 this does not appear to be practicable. In any event the 

 breeder comes as near self-fertilization as the nature of the 

 species makes advisable in order to come just as near to 

 breeding a " pure line " as possible. In actual practice 

 this never produces an absolutely pure race no matter 

 how long the pedigree. On the other hand, races are 

 often pure for individual characters which are reproduced 

 generation after generation with remarkable fidelity. 



On the Effects of Selection. — It must be clear from 

 the reader's present knowledge that recombinations (page 

 36) are continually occurring in respect to some charac- 

 ters in animals. Some of these may be slightly better and 



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