FUNCTIONS OF THE PARASITIC BACTERIA 41 



tion of the parasitic group may be explained. It is indeed 

 so active in certain individual cases as to give to its possessor 

 almost complete immunity from particular types of parasitic 

 invasion. 



To illustrate: Diphtheria is caused by a well-known 

 parasite. It has many interesting properties but the most 

 significant of its physiological activities is its power to 

 elaborate a poison that causes the group of symptoms and 

 tissue changes which we know as diphtheria. No other 

 organism has this property. This same diphtheria bacillus 

 may invade one person and cause his death, while in another 

 the results of its activities may be comparatively trifling. 

 Similar extremes of variation are constantly seen in the case 

 of all the known infective diseases. It is to be explained in 

 but one way "the soil," i. e., the living host, in which the 

 disease exciting* " seed," the germ, finds itself is not that 

 which is best suited to its active growth, or in other words 

 one individual possess higher natural powers of resistance 

 than does another, and in a large group of individuals such 

 differences in the degree of resistance are marked. We see 

 nothing like this in the action of saprophytes upon dead 

 matter. It is true we see their growth restrained at times. 

 In some cases such restraint is exercised by other species 

 the products of whose growth are antagonistic; and in a 

 number of cases the growth of saprophytes is often for a 

 time checked by the accumulated products of their own 

 activities. For instance the growth of those saprophytes 

 concerned in acid fermentations comes very quickly to an 

 end unless special provisions be made to neutralize and fix 

 the acids as fast as they are manufactured, for no bacteria 

 develop indefinitely in the presence of free acids. This is, 

 how r ever, a very different kind of inhibition from that 



