REPORT ON THE LAWS OF CONTAGION. 71' 



parently perfect insulation*. But in this and all similar cases, 

 the probability is much greater that a specific disease, like small- 

 pox, should have been received from a pre-existing source, than 

 that, contrary to all experience, the poison should have origin- 

 ated afresh. Many instances too are on record, in which the pene- 

 tration of contagious diseases,into situations supposed to be per- 

 fectly isolated, has been traced to intercourse, though forbidden 

 by the strictest rules, and even by menaced punishment. An- 

 other mode of conveying infection, beside that of direct commu- 

 nication, which will be pointed out in the sequel (§. xxii. et seq.), 

 will account for a great part of the apparent exceptions. 



VII. The conclusion that ' contagious diseases of a specific 

 kind never originate spontaneously,' is strengthened by the fol- 

 lowing facts : — 1 . They have never been met with in any coun- 

 try, when visited for the first time, after having been previously 

 shut out from intercourse with the civilized world. 2. The hi- 

 storical aeras may be fixed, when many of them first invaded the 

 countries where they now prevail, and the line of their march 

 may be distinctly traced outf. 3. Specific diseases have been 

 known to become extinct for a time in certain situations, and 

 their revival has been traced unequivocally to a foreign source. 

 Thus, the smallpox disappeared several times from the island 

 of Minorca, apparently from having already attacked all who 

 were liable to it. In one instance the interval extended to se- 

 venteen years ; in another, after having been absent for three 

 years, its return was clearly traced to the crew of a ship of war 

 which had arrived from the Levant. Seven similar intermissions 

 of the same malady are recorded to have happened at Boston in 

 New England, in three only of which the channel of its reintro- 

 duction could be discovered. But these three instances render it 

 much more probable that the poison, causing the disease, should 

 in the remaining four have been imported anew, than that it 

 should again have been generated. For though it cannot be 

 denied that a poison may be again elaborated, by a concurrence 

 of the same circumstances which originally produced it, yet, in 

 assigning causes, we must be guided by actual observations, and 

 not by possible contingencies %. 



* Fact communicated by Dr. Roget. 



t. Hawksworth's ^'byra^es, vol. iii. page 56. Siphylis was introduced by the 

 crews of Bougainville's vessels into the Sandwich Islands, ii. 232. De Pauw, 

 Recherches P hilosophiques stir IBs Am&ricains, torn. i. Robertson's History of 

 America, book iv. 



■X The origin of new specific diseases is a topic too extensive to be entered 

 upon here. The reader is referred, therefore, to an excellent essay by Dr. 

 Ferriar, in the first volume of hh Medical Histoi'ies and Reflections; to the 

 various publications of Dr. Jenner, John Hunter, Adams; &c. 



