APPENDIX E. cxix 



One of the great and distinguishing peculiarities of these 

 specific diseases is their 'contagiousness/ Although very 

 differently marked in the several affections, this property 

 is as interesting as it is important. The fact of its existence 

 seems always to have had a large share in determining the 

 nature of the general views which have been held concerning 

 these affections. Even in remote periods, by Hippocrates 

 and others, they were commonly compared to processes of 

 fermentation ; whilst, since the time of Linnaeus more espe- 

 cially, attention has been often prominently directed to the 

 many apparent similarities existing between the commence- 

 ment and spread of epidemic diseases, and the ' flight, settle- 

 ment, and propagation of the insect-swarms which inflict 

 blight upon vegetable life 1 / These analogies were seem- 

 ingly strengthened by the increased knowledge which 

 gradually arose concerning the various parasitic maladies 

 to which man and the lower animals were liable. Writing 

 in 1839, Sir Henry Holland says, in his essay 'On the 

 Hypothesis of Insect-Life as a cause of Disease/ ' The 

 question is, what weight we may attach to the opinion that 



these conditions) may be even more easily initiated by a single specific 

 cause. The introduction of a crystalline fragment into a saline solution, 

 and its determination of the crystallisation of all the isomorphous salts 

 contained in the solution, seems to be exactly comparable with the 

 ' contagious ' origin of diseases. But, under the influence of certain 

 favouring conditions, crystallisation may occur without the contact of 

 a crystalline fragment the process may be ' spontaneous ' in the same 

 sense that the occurrence of the blood-change may be ' spontaneous.' 



1 Sir H. Holland's ' Medical Notes and Reflections,' second edition, 

 1840, p. 584. On the following page, the same author writes : ' Con- 

 nected with these facts is the observation, seemingly well attested, that 

 the cholera sometimes spreads in face of a prevailing wind, and where 

 no obvious human communication is present a circumstance difficult, 

 if indeed possible, to be explained, without recourse to animal life as 

 the cause of the phenomenon. No mere inorganic matter could be so 

 transferred, nor is vegetable life better provided with means for over- 

 coming this obstacle.' Whilst on the preceding page, the ' animal 

 species' had been admitted to be 'minute, beyond the reach of all 

 sense.' 



