THEORIES IN PRACTICE 361 



tained its position for ages to come, just as it 

 had maintained it throughout the ages of the 

 past. 



Yet we must not forget that on occasion there 

 may be natural methods of elimination that will 

 single out a species and destroy it as expedi- 

 tiously and as certainly as man could accomplish 

 that end. 



A case in point is furnished by the chestnut, 

 which, as we have seen in a recent chapter, has 

 been singled out in certain regions of the eastern 

 United States by a fungoid blight that leaves no 

 chestnut alive in the regions over which it 

 spreads. Yet this blight seems powerless to 

 affect any other species. 



Here, then, we have an example of a destruc- 

 tive agency of an unpredicted kind that gives an 

 example of the rapid destruction of a species, 

 through natural selection, because that species 

 could not rapidly enough adapt itself to a new 

 condition. 



Given time, the chestnut would doubtless 

 develop immunity to the fungoid pest. But time 

 was not given it, and hence it was destroyed. 



This present-day illustration perhaps gives as 

 vivid an impression of one of the more tangible 

 ways of the operation of natural selection as 

 could be desired. But wfe must suppose that such 



