INTRODUCTORY AND GENERAL 3 



or perhaps minutes. Readers of one of Mr. Wells's ingenious 

 romances may perhaps remember how the strange beasts from 

 Mars which invaded this planet died rapidly, being evolved in a 

 region in which there were no bacteria, and in which this power 

 of resisting their action had not been developed. The example is 

 a striking one, and is strictly scientific, though we may wonder 

 how the rotation of nitrogen, in which bacteria play so essential a 

 part, is brought about in Mars ; for this process of the breaking 

 down of dead proteids by bacterial action, and the preparation of 

 its nitrogen for use in plants, is essential for continued life on the 

 planet. Without decomposition all the combined nitrogen of the 

 world would soon become locked up in the dead bodies of animals ; 

 plants would starve and die, and animals (which are all dependent, 

 directly or indirectly, on plant nitrogen) would likewise become ex- 

 tinct. It is a most marvellous natural phenomenon that these putre- 

 factive bacteria should be found wherever life occurs, and wherever 

 their aid may be required to deal with the protoplasm when dead, 

 and that this same protoplasm should have acquired such potency 

 in resisting their attacks whilst still alive. Absence of bacteria 

 or absence of immunity are alike incompatible with animal life. 



Considerations of this nature lead us to a short discussion of 

 the difference between the pathogenic and non-pathogenic bacteria, 

 and we find that there is, theoretically, none. Any bacterium 

 will produce disease if it grows in the tissues of the living body, 

 and all bacteria 1 will do so if the necessary degree and form of 

 immunity is not present. A pathogenic organism is one which 

 can grow in the living tissues, and it can do so only because those 

 mechanisms of immunity which are sufficient in the case of the 

 saprophytic bacteria are powerless to resist it ; but in most cases, 

 as we shall show, a higher degree of immunity can be produced 

 artificially, and the microbe in question then becomes non-patho- 

 genic to that particular animal. So, too, with the bacteria 

 ordinarily regarded as non-pathogenic. Under certain circum- 

 stances, some of which are known and some still unknown, the 

 resistance of the body or of a part of it may be broken down to 

 such an extent that these organisms may gain access, flourish, 

 and give rise to disease. Thus, B. proteus may give rise to 

 phlebitis, growing in the thrombosed vein, and giving off toxins 

 which have an injurious action on the tissues. 



1 Bacteria growing only at very high or very low temperatures, or on media 

 very poor in nitrogen, perhaps excepted. 



I 2 



