338 THE SURVIVAL OF THE UNLIKE. [xX. 



with the idea of a permanent modification of climate. 

 Over one hundred years ago, Hugh Williamson, 

 M. D., attempted to account for the "very observable 

 change of climate" which had taken place in Penn- 

 sylvania within the preceding forty or fifty years. 

 He declared that ' ' our winters are not so intensely 

 cold, nor our summers so disagreeably warm as they 

 have been." Most persons suppose that an opposite 

 change has taken place in recent years. 



It is, therefore, useless, in a discussion like the 

 present, to attribute the failure of apple orchards to 

 increased severity of climate. And it is also appar- 

 ently, unnecessary to do so, as other causes appear 

 to adequately explain the falling -oif. 



2. Greater abundance of insects. — There is no 

 question that insects are more numerous, both in 

 individuals and in destructive species, now than 

 formerly. The increase in individuals is due to the 

 greater number of trees grown in later years. The 

 increase of noxious species is due to the changes of 

 habit of various species, and to introductions from 

 foreign countries. But there are comparatively few 

 insects which occasion the death of the tree itself. 

 Insects which weaken the tree are mostly borers, and 

 these species appear to have been common in former 

 times ; and although their individual numbers may 

 have increased, their injuries are undoubtedly more 

 than counterbalanced by the greater pains taken in 

 destroying them in later years. Increased insect dep- 

 redations unmistakably lessen production in recent 

 years, but I cannot believe that they lessen longevity 

 of trees. Some contend that attacks upon the foliage 

 tend to lessen the vitality of the tree, and therefore 



