112 The Field Naturalist 's Quarterly May 



and explain briefly and simply the general phenomenon so 

 referred to. The exact nature of this phenomenon is one 

 of the most fascinating and difficult problems in modern 

 biology, and its solution is fraught with matters of the 

 greatest importance to the welfare of mankind. 



The first definite idea or conception of immunity was 

 formed when physicians learnt that after recovery from 

 certain diseases, and especially after smallpox, an enduring 

 protection may be secured from a particular or specific 

 disease. In some cases this protection is lifelong, in others 

 more restricted. Jenner then pointed out that a similar re- 

 sult is obtained by undergoing a nearly related condition 

 instead of the actual disease itself, as in vaccinia. In more 

 recent years Pasteur's discoveries in artificial inoculations 

 have given us a still clearer conception of what immunity 

 really means. The question gradually shaped itself definitely 

 into the query, Why is man not liable to certain diseases of 

 animals ? Why are animals never affected by some human 

 diseases ? Why do some races or individuals differ from 

 others in their susceptibility to particular diseases ? The 

 answer to these questions involves the explanation of im- 

 munity. That answer cannot yet be given in full, but the 

 putting of the question in that definite form enables us to 

 define what we mean by the term ; and so we may define 

 immunity as that condition of an individual, an organ, or a 

 tissue, in virtue of which infection can be resisted either 

 absolutely or partially. For example, it is stated by some 

 that the Mongoose can resist the poison of venomous ser- 

 pents, and that it has this power because it is a Mongoose. 

 In other words, it is asserted that this species of animal is 

 immune to snake-bite ; its tissues can resist the action of the 

 venom in virtue of some property inherent in themselves, 

 that property being what we understand by immunity. I 

 am not concerned just now with the truth of the statement, 

 I merely use it as an illustration. But if the Mongoose is 

 thus immune to snake-bite, that is obviously a very different 

 thing from the immunity that is conferred upon an individual 

 as the result of having been attacked by a disease. Here 

 some new element is concerned ; the person who was before 

 susceptible has become immune. So that we see at once im- 



