CHAPTER XII. 



CAUSES OF DISEASE. THE LIVING 

 ENVIRONMENT. 



Causes due to animals Vertebrata IVoufids, etc. 

 Invertebrata Insects, etc. -Plants as causes of dis- 

 ease Phanerogams, iveeds, etc. Cryptogafns, fungi 

 Epidemics, etc. 



Passing now to those causes of disease which are 

 connected with the living environment, we may 

 obviously divide them into two groups of agents, 

 animals and plants. 



Among animals, the various vertebrata, includ- 

 ing man, are especially responsible for the larger 

 kinds of wounds and wholesale destructive pro- 

 cesses due to breakage, stripping of leaves and 

 bark, cutting and biting, and so forth. Cattle, 

 rabbits, rats and mice, squirrels and birds of 

 various kinds stand out prominently as enemies to 

 trees and other plants, to which they do immense 

 injury in various ways by their horns, teeth, claws, 

 and ^eaks ; and the damage which an ignorant 



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