296 t ELEMENTS OF GENERAL SCIENCE 



for plants upon which rabbits feed. The supply of food may 

 become exhausted, and starvation of the rabbits may result 

 largely because of the success of the large number of rabbits 

 in escaping the animals that prey upon them. 



The environment changes, therefore, for wolves and foxes, 

 for rabbits, and for the plants upon which rabbits feed, but 

 these are only a small part of the factors that make up the 

 natural environment in any region. The whole matter is ex- 

 tremely complex and is constantly changing. This constant 

 change in the environment means constant change in the kinds 

 of things that may live. Variants that can live in one environ- 

 ment may sometimes be unable to live in the same region 

 when it has undergone extensive changes. 



322. Results of natural selection. We have seen from our 

 studies of overproduction and variation that many variable 

 forms are introduced upon the earth, most of them failing to 

 live to adult size and failing to perpetuate their kind. The 

 living plants and animals now found are to be looked upon 

 as the few that have persisted. Not only this, but natural 

 selection is still going on, and we must not look upon the 

 things now living as the final types of inhabitants of the 

 earth. They are merely the results of nature's processes at 

 this time, and nature is still working. 



323. Artificial selection. Man selects those variations that 

 give promise of being advantageous to him in some way. He 

 tries to adjust the environment about these plants and animals 

 so as to produce the best results. If he wishes to produce an 

 animal that will furnish beef in large quantities, lie selects and 

 grows the variety which most nearly resembles his ideal. He 

 then tries to create the most favorable environment by careful 

 feeding, pasturing, and housing, and by warding off disease. 

 In the same way sheep, swine, driving horses, corn, wheat, 

 coffee, carnations, and roses are selected and grown under 

 artificial conditions, and results are produced which are often 

 very unlike those found in undisturbed nature. 



