68 FOURTH REPORT — 1834, 



any assistance from the most refined instruments, or from the 

 most delicate chemical tests. All that we perceive is a series of 

 events, often faintly marked, the connexion of which with each 

 other, even their order as to priority or sequence, can only be de- 

 duced by processes of reasoning, that are open to more than usual 

 sources of fallacy. In no one instance is the effect of an external 

 agent upon living animals universally the same, but modified by 

 peculiarities of structure; by temperament, age, sex, and habit; 

 and above all, by those imperceptible changes to which the ner- 

 vous system is perpetually liable. Even our mental constitution 

 and habits, — the imagination, the affections, and the passions, — 

 exercise a powerful sway over our susceptibility to contagious 

 diseases ; and when such diseases do arise, often direct their 

 course and determine their issues. The phsenomena of con- 

 tagion, moreover, are in many cases extremely complex, being 

 owing to a variety of causes which it is far from easy to ana- 

 lyse, and separately to weigh and appreciate. The omission, too, 

 of a single link in a chain of observations has frequently ren- 

 dered the whole series valueless, as data for accurate reasoning. 

 Difficult, however, as the investigation is in itself, it has been 

 rendered still more so by the manner and temper in which it has 

 been conducted. Every kind of error, that has obstructed the 

 progress of philosophy, may be exemplified from writers on this 

 subject. Observers have viewed phsenomena with the desire of 

 establishing preconceived opinions. Facts have been described 

 in language so highly coloured, or so mingled with hypotheses, 

 that it is scarcely possible to discover its legitimate meaning. 

 All that favours one side of an argument has been strongly in- 

 sisted upon, while adverse evidence has been denied its due au- 

 thority; and the love of truth has been sacrificed to the anxiety 

 to baffle an adversary by ingenious sophistry. Such at least is 

 a faithful picture of the greater part of what has been written 

 on this subject in the spirit of controversy, excited, as it has 

 generally been, by intemperate discussions of the quarantine 

 laws. But it would be unjust not to except from this censure 

 a numerous class of writers on contagious diseases, who have 

 united an eminent capacity for observing and reasoning, with 

 perfect singleness of purpose in tlie pursuit of truth. The names 

 of Lind, Pringle, Cleghorn, Russell, Blane, Haygarth, Willan, 

 Currie, Ferriar, and of many others who might be enumerated, 

 are sufficient pledges for the accuracy of their reports of facts, 

 and for the soundness of their conclusions. It is to authorities of 

 this kind (in many instances confirmed, in a few corrected, by my 

 own observation,) that I am chiefly indebted for the materials of 

 the following pages, to which I have given the form of proposi- 



