PAKENT AND OFFSPRING 389 



square miles, the environment for rabbits is greatly altered. 

 Under such conditions many rabbits can live which could 

 not live if the wolves and foxes were present in their usual 

 numbers. Thus the changed environment, so far as these 

 factors are concerned, is favorable to the life of a larger 

 number of rabbits. 



But what do rabbits live upon ? In one region in which 

 they live in winter they eat large quantities of buds from 

 low shrubs (as young trees of sassafras, hawthorn, etc.). 

 An unusual increase of rabbits thus greatly affects the envi- 

 ronment for plants upon which rabbits feed. The supply of 

 food may become exhausted, and starvation of the rabbits 

 may result largely because so many of them have succeeded 

 in escaping 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. This constant change in 

 the environment means constant change in the kinds of things 

 that may live. Variants that can live in one environment may 

 sometimes be unable to live in the same region when it has 

 undergone extensive changes. 



400. 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 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. 



401. 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 



