6 THE SCOPE OF THE ENQUIRY [CH. i 



moisture, with or without production of gas or odour ; power 

 to liquefy gelatin, to reduce neutral red, to clot milk, to ferment 

 various carbohydrate substances : power to form agglutinins 

 and susceptibility to agglutination : viability under diflferent 

 conditions : virulence or the nature of the toxins they produce : 

 pathogenicity or the nature of the lesions they cause and the 

 kind of animal susceptible to their invasion. 



It is seen from this list that the characteristic qualities o# 

 bacteria are very numerous and it would be thought that their 

 classification would on this account be very thorough and 

 complete. 5it, as will be shown in the course of this enquiry, 

 every one of these characters is liable to variation and the 

 occurrence of these variations renders the task of classification 

 very difficult and in many cases uncertain. 



If certain characteristics were invariable, even though 

 others varied, a definite criterion would be afforded, but where 

 all alike are subject to modification the division into species 

 is necessarily an arbitrary one. Amongst the higher animals, 

 where sexual production prevails, mutual fertility or sterility 

 offers a guide in determining the limits of species. Here no 

 hard and fast line can be drawn. Nevertheless we see exhibited 

 amongst bacteria, in the words of De Bary,'"the same periodi- 

 cally repeated course of development within certain empirically 

 determined limits of variation," which is considered to justify 

 the recognition of a species. 



Many species of bacteria do show characters apparently 

 quite fixed and rigid. The anthrax bacillus and the tetanus 

 bacillus are quite as good species in the natural history sense 

 as any that can be found amongst flowering plants. But the 

 classification of others is still a matter of dispute. This can 

 be illustrated by reference to the streptococci. 



The Classification of the Streptococci. 



Marmorek held the opinion that the human streptococci 

 constituted one species. "He based his chief argument on the 

 observation that bouillon in which one sort of streptococcus 

 had grown would not serve afterwards as a culture medium 

 for any other streptococcus, and that the same haemolytic 



