cxxxvi THE BEGINNINGS OF LIFE. 



of organisms have signally failed ; these fevers, indeed, pre- 

 vail in the most variable sites, and are by no means restricted 

 to marshy districts. Dr. Fergusson says : ' The first time I 

 saw intermittent and remittent fever become epidemic in an 

 army was in 1794, when, after a very dry and hot summer, 

 our troops, in the month of August, took up an encampment 

 at Roosendaal in South Holland. The soil was a level plain 

 of sand with perfectly dry surface, where no vegetation 

 existed or could exist, but stunted heath-plants. On digging, 

 it was universally found percolated with water to within a 

 few inches of the surface, which, so far from being at all 

 putrid, was perfectly potable in all the wells of the camp.' 

 These diseases, under ordinary circumstances, are most 

 certainly not contagious, and yet all the best authorities on 

 the subject are agreed that yellow fever, which is capable of 

 being propagated by contagion under circumstances favourable 

 to its extension, is but an aggravated form of remittent fever, 

 as it occurs in warm countries *. This gradual conversion 

 of a non-contagious into a contagious form of disease, com- 

 bined with the limitations as to the nature and degree of its 

 contagiousness, which the widest experience compels us to 

 accept, are facts of the utmost importance for those who seek 

 to learn the nature and origin of the contagious influence. 

 And, as almost similar limitations have to be accepted with 

 regard to the contagiousness of cholera and dysentery, it is 

 of the greatest importance to ascertain the nature of these 



1 ' On Marsh Fever,' in ' Edinburgh Philosophical Transactions,' vol. 

 ix. p. 274. And yet, concerning this disease, Dr. Milroy says : 'That 

 yellow fever is constantly making its appearance, at intervals more or 

 less distant, in various tropical countries, quite independently of any 

 suspicion of antecedent importation, just as malignant cholera does in 

 Hindostan, does not admit of doubt. In some seasons, from causes 

 which we have hitherto failed to discover, it exhibits a much greater 

 diffusion and migratory power than in other seasons. . . . Malignant 

 cholera is much more diffusible and migratory than yellow fever ; few 

 regions of the world have escaped its assault.' 



