NATURE OF BACTERIOPHAGE 331 



two fundamental properties, — the power of true assimilation, or assimila- 

 tion of a heterologous medium, and the power of adaptation. We term 

 all beings living which reveal to us that they are endowed with these 

 properties; we call those inert in which these properties can not be 

 detected. Moreover, these two powers are always found in association, 

 hence they must both be expressions of the same physico-chemical 

 property of matter. 



Let us consider the two parts of this combination of properties. A 

 crystal of copper sulfate, for example, immersed in a concentrated 

 solution of copper sulfate increases in volume; it adds to itself molecules 

 of the copper sulfate present in the solution. Is it living? No. For 

 if one wished to explain this aggregation of molecules by terming it an 

 assimilation, it would at once be obvious that the change is brought 

 about in a homologous medium. 



Assimilation in a heterologous medium implies the power of modify- 

 ing the heterologous substances contained in this medium rendering 

 them comparable to the distinctive substance of the being which brings 

 about the transformation. 



If the being, whose nature is under consideration, is visible macro- 

 scopically, it is, in general, not difficult to ascertain whether or not it 

 possesses the power of assimilation in a heterologous medium. We 

 observe a lamb and we see that it feeds on grass and increases in size; 

 we see that it gives birth to other lambs, and we know that of necessity 

 they must have been formed of the grass. W^e know that the lamb 

 transforms "grass substance" into "sheep substance." It possesses the 

 power of assimilation of a heterologous medium : it is living. 



If the being whose nature we wish to determine is microscopic we say 

 that it is living if it is able, just as was the case with the sheep, to assimi- 

 late a heterologous medium. We find that the bacterium multiplies in 

 a man, for example, and we say that it transforms ''man substance" 

 into "bacterium substance." It possesses the power of assimilating a 

 heterologous medium, and, therefore, by definition, it is living. 



If we are dealing with a being of such minute size that it is impossible 

 to see it even with the microscope, does this fact render the problem of 

 its nature beyond solution? Is it impossible to ascertain whether or not 

 it is capable of assimilating a heterologous medium? Not at all. For 

 in no case do we see the actual assimilation; we observe only the effects 

 of the process. The being increases in size or it multiplies in the heterol- 

 ogous medium, and that is sufficient to justify the conclusion that it 

 possesses the power of assimilation. 



