NATURE OF BACTERIOPHAGE 351 



lus of the same nature as the first fails to occur until after the lapse of 

 a long time the "recollection" of the reaction effected is progressively lost. 



The living being reacts continually. Since the conditions of the 

 medium in which it lives are changing with each moment, and in very 

 diverse ways, the specific reactions which it brings into play must adapt 

 themselves to innumerable conditions. And the "recollection" of 

 each of them imprints itself in the substance more or less strongly 

 according to the energy expended in each of these reactions, and each 

 "recollection," intense or mUd, as the case may be, is transmitted to 

 the descendents. As the conditions are different for each being of a 

 single species, that is to say, for a single line, it results that each being 

 presents distinctive characters and special aptitudes, either forced upon 

 it through heredity or acquired through its own experience. 



The less complex a being the greater is its faculty of adaptation. 

 This fact can be experimentally demonstrated, for of the visible beings 

 the bacteria have the most simple organization and we know that it is 

 possible to induce them to develop under very diverse conditions, some 

 extremely remote from their natural conditions of life. The greater 

 the adaptabihty, the greater also is the variability. Indeed, when bac- 

 teriology was in its infancy, Pasteur showed that each bacterium within 

 a single culture presented, together with characters belonging to the 

 species, individual characters and aptitudes for adaptation. 



Does the bacteriophage show this variability? The experimental data 

 recorded on each page of this text have shown that there are no two 

 races of the bacteriophage which are identical. Within a single suspen- 

 sion each corpuscle has its own individual virulence, and each presents 

 an individual resistance to the action of physical or chemical agents. 

 Furthermore, the experiments of Prausnitz and Firle have demonstrated 

 that each corpuscle within a single suspension has a capacity for adapta- 

 tion differing in degree from that of other corpuscles. 



Some experiments of Wolff and Janzene^o dealing with variability 

 are particularly interesting, being concerned with the variation occur- 

 ring in the virulences when a given bacteriophage multiplies at the 

 expense of bacteria of different species.* 



* It may be mentioned that these experiments might have been cited to demon- 

 strate the autonomy of the bacteriophage corpuscle as regards the bacterium. 

 When different bacteriophages develop at the expense of a single bacterium, they 

 should, if they are a product of bacterial metabolism, all tend to assume the same 

 characters, in the present case to acquire the same virulences. But experiment 

 shows the contrary. The aptitude to lose or to acquire a virulence does not de- 

 pend on the bacterium "lysed" ; this is a character appertaining to an autonomous 

 entity. 



