As East and Jones point out, "this reduction applies 

 only to the whole population, or to a representative sample 

 of the population, in which every member is selfed, in which 

 each individual is equally fertile, and in which all the pro- 

 geny are grown in every generation. In practice in an in- 

 breeding experiment, usually only one individual in self- 

 fertilization or two individuals in brother and sister mating 

 are used to produce the next genertion. 



Thus the rate at which complete homozygosity is 

 approached depends on the constitution of the individuals 

 chosen" {Inbreeding and Outbreeding). 



Chapter 18— A COMPARISON OF PLANT AND 

 ANIMAL BREEDING 



The plant breeder has an advantage over the animal 

 breeder since he can handle thousands of individuals to one 

 by the latter. In selection he is, therefore, better able to 

 find variant forms, even mutations, that suit his purpose. 

 Moreover, in hybridizing better opportunities are offered 

 for the segregation of the unit characters. 



The factors of rapidity of production, the possibility of 

 asexual reproduction, and the relative cheapness of individ- 

 uals are also important advantages in favor of the plant 

 breeder. 



On the other hand, the fact that plants are influenced to 

 a greater extent by soil and climate than animals are, and 

 that it is more difficult to keep actual records in the case of 

 plants, places the plant breeder at a disadvantage. 



The plant breeder endeavors to produce new breeds or 

 races, different in some important characters from their 

 type. On the other hand, the animal breeder does not 

 strive after the production of new breeds or races, but rather 

 "improvements within the breed or race, such as increased 

 size, greater milk production, improved beef quality, in- 

 creased fecundity, or some such quality." 



(a) — Animal Improvement 



The various pure-breeds among horses, cattle, sheep, 

 swine, etc., have arisen from native stock by selection and 

 prevention of mixing with inferior strains. (See pages 

 33-34). 



In recent years the introduction of Pedigree Registers 

 has done much to keep the breeds pure. Such Registers 



152 



