CHAPTER XXI. 



DISEASES AND INSECTS. 



Diseases. The laws of health and- disease 

 are very much the same in animal and vegeta- 

 ble life : plants, in common with man, will be- 

 come liable to disease by an infraction of these 

 laws. The subject is so broad that we can only 

 treat it in a general way here. We wish to es- 

 tablish the analogy, however, since it will do 

 away many illusions in the mind of the novice. 

 Different kinds of animals have their allotted 

 periods of life : in one kind it may be ten years 

 or less, while in others it may be fifty or more ; 

 the elephant, for example, lives to a much 

 greater age than the dog. .It is so with plants : 

 some fulfill their life in a single year, while 

 others count " the years of their life" by thou- 

 sands. The average life of plants is greater 

 than that of man. The existence of both is 

 shortened by violence in various forms, and 

 both are liable to disease. The average health 



