cxlviii THE BEGINNINGS OF LIFE. 



frequently concerned in their production. There are the 

 'predisposing causes' pertaining to the condition and ten- 

 dencies (either inherited or acquired) of the individual, and 

 there are the ' exciting causes ' or external influences (usual 

 or unusual) at the time operative upon this individual. The 

 combined influence of these causes of disease are often 

 called into play in the production of the infective malady, 

 just as much as they are influential in the origination of 

 non-infective diseases. But predisposing causes may, in 

 conjunction with ordinary external agencies, suffice in some 

 cases ; just as, in other cases, the exciting cause or causes 

 may be capable of initiating the affections in the average 

 healthy individual, without the aid of any predisposition. 



Unless we entertain opinions of this kind, facts, which are 

 admitted by all, seem quite incapable of being explained — 

 whether having reference to the ' generalisation' of morbid 

 growths within the body, or to the spread of infectious dis- 

 eases amongst the community. Cancerous particles in the 

 circulation are wholly inoperative in certain individuals in 

 inducing the growth of cancer in distant parts, whilst they are 

 only partially operative in many other individuals, however 

 numerously they may exist. Contact with the contagia of 

 ophthalmia or diphtheria will excite the disease in some per- 

 sons and not in others. Yellow fever and cholera are ' conta- 

 gious ' only when certain favouring conditions are present to 

 facilitate the operation of the specific poisons of these diseases. 

 Rabies cannot be communicated to certain dogs. Professor 

 Gamgee ^ mentions a case in which a pointer did not con- 

 tract the disease although it was bitten seventeen times by 

 mad dogs. And even the most contagious affections — 

 those in which the poison is usually sufficiently potent to 

 act upon the average individual — do not seem capable 

 of being communicated to some persons. Do we not see 



■^ In Reynolds's 'System of Medicine,' vol. i. p. 717. 



