138 TEXT-BOOK OF BACTERIOLOGY. 



occurrence may perhaps be convenient and therefore permissible. 

 Yet we must always keep in mind that by adopting 1 such a course 

 we may easily lose the solid ground under our feet, and find our- 

 selves all adrift with our artificially-built-up system of differential 

 diagnostic criteria. 



As will hereafter be recorded, we are acquainted with quite a 

 number of micro-organisms, evidently closely allied to each other, 

 which slight indications only enable us to distinguish from each 

 other and to arrange in definite groups. Wherever the attempt 

 has been made to employ differences of virulence or infective 

 power as grounds of separation within these definite groups, it has 

 always been found, sooner or later, that this proceeding yields no 

 reliable results, and that it is useless in the long run. 



When it is understood that the pathogenic qualities of the bac- 

 teria are their least constant qualities, that from clearly intelligible 

 causes they form the most variable item in the picture of micro- 

 organic appearances, we shall be very little inclined to cite varia- 

 tions in this particular as arguments against the law of constancy 

 of species before alluded to. In fact, it has hitherto, in every case, 

 been possible to recognize permanently as a separate species those 

 micro-organisms which have been once recognized as such. It is 

 true that, under some circumstances, a very careful and precise 

 consideration of all their peculiarities is necessary : yet where this 

 condition has been fulfilled one species has never been found to 

 merge into another and display its characteristic marks. 



In conclusion, one more point will be considered, which grows' 

 naturally out of what has just been written. 



That the virulence of a given micro-organism may, under natu- 

 ral conditions, be sometimes greater, sometimes less, we have al- 

 ready seen. This fact has been, with great probability, set down 

 as one of the reasons why the same infectious disease often occurs 

 with different degrees of malignancy, a phenomenon which would 

 otherwise be very striking. It is known that the higher plants 

 show something similar; that, for instance, the fox-glove from a 

 given locality will yield in one year a very potent poison, in the 

 next year a very much weaker one. Now, if diphtheria bacilli of a 

 peculiarly virulent kind infect a human being and pass over from 

 individual to individual, at length causing an extensive epidemic, 

 will not, or cannot, its character depend on the original qualities 

 of the disease-exciting micro-organism ? 



These considerations might be spun out to a considerable length 

 in various directions. But we will not exhaust ourselves in theo- 

 ries nor run on in advance of our real knowledge. It will not have 



