366 THE BACTERIOPHAGE AND ITS BEHAVIOR 



same way to B. coli as to a superior plant, Melilotus altissima, for ex- 

 ample? It is needless to enter here into comparisons such as anyone 

 may make; to do so would but divert us from the more important 

 subject. 



We are forced to conclude that with these rudimentary beings, the 

 bacteria, and unquestionably this is fully as true with the protobes, 

 the concept of species can not have the same meaning as with beings 

 higher in the scale of organization. As a matter of fact, if one wished 

 to generalize, he would be forced to admit that a bacterium changes its 

 species according to the conditions of the moment. A striking example 

 is given us in the paper cited in the preceding section. Brocq-Rousseu, 

 Urbain and Forgeot, after stating that "the strangles streptococcus is a 

 species which can be differentiated from all of the other known strepto- 

 cocci," add, "by passages through animals, the strangles streptococcus 

 loses its characters." Certainly, these two statements do not harmon- 

 ize. It would be only too easy to multiply examples of contradictions of 

 this kind. 



4. THE UNICITY OF THE SPECIES PROTOBIOS BACTERIOPHAGUS 



The fact that an "ultrapure" bacteriophage may show multiple 

 virulences, that is to say, may parasitize bacteria belonging to different 

 species, sometimes bacteria very remotely related, indicates that there 

 is but a single species of the bacteriophage. Experiment shows further 

 that this being is capable of adapting itself to the parasitism of bacteria 

 which it did not previously invade. This reahzes experimentally a 

 process which is continually taking place in nature. 



But if it is true that there is but a single species of bacteriophage, 

 within this rudimentary being the phenomena of adaptation go on con- 

 tinually, the variabihty attained is extreme, and the point is reached 

 where each race possesses its own characters (d'Herelle^^^). 



This should not surprise us. The situation is exactly the same with 

 the neighboring beings, the bacteria. We know very well that within 

 any bacterial species there are no two strains which are, or can be, 

 identical. 



Objections to the concept of the unicity of the bacteriophage have 

 been raised by a number of authors, but before discussing these objec- 

 tions I should first mention that none of them have explained just what 

 they mean by "plurahty of bacteriophages." If they mean that each 

 strain, each race* of the bacteriophage possesses its own characters, 



* Throughout this text the word "race" has been used intentionally in speaking 

 of the bacteriophage, while the term "strain" has been reserved for bacteria. 



