CHAPTER XVI. 



THE FACTORS OF AN EPIDEMIC. 



Illustrations afforded by the potato disease The larch 

 disease The phylloxera of the vine. 



When we come to enquire into what circum- 

 stances bring about those severe and apparently 

 sudden attacks on our crops, orchards, gardens, 

 and forests by hosts of some particular parasite, 

 bringing about all the dreaded features of an 

 epidemic disease, we soon discover the existence 

 of a series of complex problems of intertwined 

 relationships between one organism and another, 

 and between both and the non-living environ- 

 ment, which fully justify the caution already 

 given against concluding that any cause of disease 

 can be a single agent working alone. 



The statement of prophecy that a particular 

 insect or fungus need not be feared, because it is 

 found to do so little harm in particular cases or 

 districts examined, will thus be seen to be a 



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