358 EVOLUTION AND GENETICS 



Summary. Hybridization and selection have played an impor- 

 tant part in the establishment of varieties of plants and animals 

 of use to man. Hybridization is useful for two purposes, \az., 

 increase in vigor and productiveness, called heterosis, and the 

 combination of useful characters. Its effectiveness in animals is 

 limited by the segregation which occurs during reproduction. 

 This necessitates repeated hybridization whenever the hetero- 

 zygous condition is the one desired. In plants even this limitation 

 is offset by the fact that asexual propagation is usually possible. 

 Hybridization is also hmited where sexual reproduction is unavoid- 

 able by the frequent infertility of hybrids. Selection is accom- 

 plished by several methods, all directed toward the isolation of 

 the most favorable individuals as the parents of the following 

 generation. It is practiced in connection with h5d3ridization for 

 the isolation of desired combinations of characters, and within 

 established species for the isolation of the best strains. Selection 

 is apparently limited by the pure lines of which a species is com- 

 posed, although the isolation of these lines may constitute an 

 effective degree of modification. Even pure lines are known to 

 be susceptible to some modification. 



REFERENCES 



Babcock, E. B. and Clausen, R. E., Genetics in Relation to Agriculture, 2nd 

 edition., 1927. 



An extensive bibliography is published in Babcock and Clausen's work. 



