28 THE BIOLOGY OF BACTERIA 



strated beyond all doubt the enormous part played by the medium 

 or soil in which the micro-organism is growing.* 



Specificity of Bacteria 



A species may be defined as a group of individuals which, however 

 many characters they share with other individuals, agree in present- 

 ing one or more characters of a peculiar and hereditary kind with 

 some certain degree of distinctness.f There is no doubt that separate 

 species of bacteria occur and tend to remain as separate species. 

 Bub it must be remembered that species are merely arbitrary divisions 

 which present no deeper significance from a philosophical point of 

 view than is presented by well-marked varieties, out of which 

 they are in all cases believed to have arisen, and from which it is 

 often a matter of individual opiuion whether they shall be 

 separated by receiving a specific label. What degree or character 

 of variation shall be considered as sufficient for the demarcation of 

 a species of bacteria ? B. coli and B. typhosus have certain distinctive 

 features, which are accepted as factors of provisional differentiation. 

 But they have many points in common, the peculiarity and heredity 

 of which are not as yet determined. And they have many allies, 

 para-typhoid and para-colon organisms, in the same way as the 

 tubercle bacillus possesses many allies, both bovine and human, among 

 acid-fast species having similar characters but differing in degree of 

 virulence. The fact is, that our present knowledge of these matters 

 is very small, and it is impossible to dogmatise. The future may 

 reveal some unlooked-for relationships, and organisms now classified 

 as morphologically separate may ultimately prove to be nearly 

 related. Further, it may be found that their respective action 

 in the human body is not greatly dissimilar (the production, 

 of diarrhcea, for example, by the colon group). Medium and 

 tissue have their effect in the production of variations of greater or 

 lesser mark in bacteria. B. typhosus may, in the course of sub- 

 culture, become morphologically indistinguishable from B. coli, and 

 its pathogenicity may also be reduced. The tubercle bacillus in old 

 culture and in saprophytic existence becomes almost indistinguish- 

 able from the streptothrix family. Streptococcus conglomeratus on 

 certain media simulates in a marked degree the Klebs-Loffler diph- 

 theria bacillus, and by passage through a mouse loses its streptococcal 



* The writer has been impressed in particular as to the truth of this view by- 

 observation of a number of epidemics, by the study of a long series of cultures of 

 the same bacillus on different media, and by antitoxin production. But the same 

 conclusion has been reached from other premises. See a suggestive paper by Sir 

 W, J. Collins, M.D., in the Jour, of the Sanitary Institute 1902 (Oct.), xxiii., pt. iii., 

 p. 335. 



t Danoin and After Danoin, G. J. Romanes, F.R.S., vol. ii., p. 231. 



