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KLK. MK NTS OF (JKNKUAL SCIKXCK 



Among plants and animals many hybrids are constantly 

 being produced. It is recognized that hybridization presents 

 an important method of production of variations. 



320. Natural and artificial selection. Whenever variations 

 occur we may in a measure determine which variants shall 

 have a chance to reproduce their kind. In the next generation 

 variations will again appear and selection will again be neces- 

 sary if we desire to perpetuate the type which has been selected. 



FIG. 136. Hybrids between a zebu and Hereford cattle 



The humps and horns in some cases show relationship to the zebu, but coloring 



and general conformation of body are usually like those of the Hereford. Some 



of the hybrids, like the zebu, are free from the tick, which carries Texas fever. 



Photograph by Robert K. Nabours 



It is possible to make a race of plants or animals less variable 

 by selecting constantly for a certain type, and by eliminating 

 the variations which do not conform to that type. 



Man decides which of all the variations he desires for his 

 own use and then tries to maintain these particular types and 

 eliminate the others. This constitutes artificial selection. In 

 open nature, through natural selection, those things that can 

 live are the ones that do live. The animals of the extremely 

 cold countries vary in the thickness and heat-retaining qualities 



