EVOLUTION 



son of obstacles in its way, but it would not vary at 

 all were it not for the push of the water behind it. 

 The inherent tendency to vary implies an inherent 

 force or effort. Only living beings struggle; it is the 

 characteristic of life to struggle, to push on. The 

 mechanical forces do not struggle, they clash and 

 seek repose; vital forces struggle and seek ascend- 

 ancy; the body struggles against disease germs, and 

 the germs struggle against the body; it is a trial of 

 strength. 



What natural cause keeps any species of animal 

 in check, or accounts for its superabundance, is a 

 question not easily answered. Take our common 

 red weasel, for example: it is very hardy and pro- 

 lific; it has few or no natural enemies; it is active 

 and fearless; it can climb trees and is nimble on 

 the ground; it preys upon a great variety of small 

 rodents and birds, having such power over them 

 that it overtakes and kills the rabbit, which is much 

 more fleet of foot; and yet it is one of our rare 

 animals; one would say there are hundreds of 

 rats, chipmunks, red squirrels, and rabbits, to one 

 weasel. Though I live in the country and spend much 

 time in the fields and woods, I do not see on an 

 average one weasel in a year. What keeps them in 

 check? Some, to me, unknown factor. In our coun- 

 try the flesh-eating animals are rare in comparison 

 to the non-flesh-eaters, or to the miscellaneous 



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