Introduction . x xix 



seminating error, which the obstinacy engendered by the evidence of 

 imperfectly observed facts has tended to contirm and to perpetuate ; 

 but it is possible to manifest too indiscriminate a contempt for state- 

 ments which partake of popular superstition.' 



Vicq-d' A.zyr truly says, that if there is in medicine an object worthy 

 the investigation of scientific men, it is without contradiction the pesti- 

 lential epidemic diseases. Obscure and often mysterious in their causes, 

 rapid in their progress, perplexing in their symptoms, and murderous in 

 tlieir effects, they often sweep away the majority of the individuals 

 attacked, and through their violence put it beyond the power of the 

 physician to diminish the number of victims. And the illustrious 

 Hecker justly affirms that the study of epidemic diseases ' is a subject 

 in which science is deeply interested, and which, according to the 

 direct evidence of nature herself, is one of the most exalted and im- 

 portant that can be submitted to the researches of the learned. How 

 often,' he adds, ' has it appeared on the breaking out of epidemics, as 

 if the experience of so many centuries had been accumulated in vain. 

 Men gazed at the phenomena with astonishment, and even before they 

 had a just perception of their nature, pronounced their opinions, which, 

 as they were divided into strongly-opposed parties, they defended with 

 all the ardour of zealots.' 



The study and prevention of animal scourges, as we have seen, is 

 scarcely second to those affecting our own species, but they are attended 

 with even greater difficulties. The healer of men, consulted when a 

 pestilence is raging, and when death is seizing numberless victims, can, 

 as we are all too painfully aware, affbrd but little aid. The rapidity 

 with which the disease does its work, and its generally obscure nature, 

 throws him into a sea of doubt, from which he can but slowly, if at all, 

 extricate himself. He who ministers to the ailments of animals, and who 

 ordinarily has to contend with obstacles to which the other is a stranger, 

 is seldom in a better plight when a formidable spreading disease visits 

 one or more species. Those people among whose herds a malady ot 

 this kind first appears are too often the opposite of intelligent, and 

 usually see in its invasion the simple effects of some vulgar cause which 

 they imagine can be easily determined j while in the death of their cattle 

 they are only conscious of a local and individual loss, far from involv- 

 ing the most insignificant interests of their country. 



As in man, when general diseases or 'plagues' first appear in llu' 

 lower animals, they are usually very acute, and in consecjuence of this, 

 of the suddenness of their attack and the rapidity of their course, as 

 well as their tendency to spread, it is a matter of the utmost import- 



