UNICITY OF BACTERIOPHAGE PROTOBE 365 



In brief, then, these experiments, together with a number of facts 

 previously known, show that assimilation in the bacteria is not complete. 

 The substance of the bacterium varies according to the foodstuff utilized 

 for its development, and this results in a variabihty of antigenic 

 properties. 



Is this also true for the protobes? This is a question which bears 

 directly upon the subject of the unicity of the bacteriophage. We will 

 consider this very shortly. 



3. THE CONCEPT OF SPECIES AMONG THE MICROBES AND THE PROTOBES 



Man has a tendency to generahze. Our knowledge of assimilation 

 has been acquired by the study of superior animals, and it has been 

 extended as such to all living beings. We have seen that this is cer- 

 tainly not correct in so far as bacteria are concerned. It is the same with 

 regard to the concept of species, which, also, has been acquired in the 

 first place by a study of superior animals and plants. To transfer 

 this concept of species to rudimentary beings and to apply it in all of 

 its details is again to follow a false trail. 



Observation of nature shows that the facility with which adaptation 

 takes place is directly proportional to simplicity of organization. With 

 the superior animals adaptation takes place only within very narrow 

 limits; as soon as a change, if pronounced and sudden, occurs in the 

 conditions of the medium the being succumbs. And since variability 

 is necessarily related to the faciUty of adaptation, the result is a high 

 degree of homogeneity among the animals or the plants belonging to a 

 single species. In so far as animals are concerned, and they have been 

 more extensively studied from this point of view, the antigenic char- 

 acters are identical in all of the individuals belonging to a single species. 

 Indeed, a common antigenic character may extend beyond a single 

 species, manifesting itself in neighboring species, a fact which has been 

 utilized to disclose the bonds of relationship uniting them. This 

 question of antigenic specificity in the animal and vegetable series 

 offers a most interesting subject for study. But however great this 

 specificity may be in other species, we have seen in an earlier section 

 that with the bacteria the characters vary even within the individual 

 according to the nature of the food. This variability in the substance 

 of a single being involves as a consequence a variability in all of the other 

 characters. What becomes, then, of the idea of species among these 

 beings? 



To take a concrete example. May we apply the idea of species in the 



