SELECTION 



25 



have been discovered it is necessary that he should make a wise 

 selection among them to attain the results desired. 



In nature selection is going on constantly, but less carefully. 

 The struggle for existence in a dense bed of seedlings of field 

 plants causes some natural selection. The most thrifty individuals 

 will reach light and air above the others and soon dwarf or kill 

 the weaker. In an arid region a thorny cactus has prevailed over 

 the spineless kinds because animals do not molest the former. 

 Volumes are filled with such examples of natural selection. 



Selection by man is more rapid than in nature. It is an im- 



FIG. 17. Vegetable trial grounds, Potomac river flats near Washington. Varieties 

 and new strains are here tested by the United States Department of Agriculture. Many 

 large seed companies test new varieties of vegetables and flowers in this way. (U. S. D. A.) 



provement upon nature by the application of the intellect of man 

 to natural laws. Certain valuable results may be attained within 

 a few generations which might require centuries if left to nature 

 (Fig. 18). Under natural conditions certain valuable types might 

 be entirely submerged, but under the guidance of the breeder they 

 may be protected and established as fixed types. 



To be successful through the medium of selection the breeder 

 of plants or animals should keep in mind the following elements: 

 (1) He should keep his ideals or amis clearly in mind. (2) He 

 should know the laws of nature governing variation. (3) He 



