332 THE BACTERIOPHAGE AND ITS BEHAVIOR 



The power of adaptation may be defined as the faculty possessed by a 

 being to respond by a specific opposing reaction to variations in the 

 conditions of the medium. 



This is the sum and substance of the question of the fundamental 

 properties of hving matter. Does this combination of properties con- 

 stitute the "criterion" of life? We may reply in the affirmative, for 

 all beings which man has classed in the category of beings which he terms 

 "living" possess this combination of the two characters, while all of 

 those which he allocates to the category termed "inert" lack it. When 

 one of these beings which we term "hving" is placed under conditions 

 suited to the manifestation of these criteria and it no longer exercises 

 these properties we feel warranted in refusing to consider it longer as 

 being hving; we say that it is "dead." For example, we heat a bacterial 

 culture at a given temperature, and we implant a drop of the heated 

 liquid into some fresh bouihon. If we find that the bacterium utiUzes 

 the nutritive substances of the bouillon for its development, if, in a word, 

 it "assimilates," we say that it has not been killed. We affirm that it is 

 still living. If, on the contrary, it does not "assimilate" we say that it 

 is dead.* 



If anyone refuses to admit that there is a "criterion" of life, and that 

 this criterion is constituted of a combination of the powers of assimila- 

 tion and of adaptation, all discussion as to the nature of any being 

 whatsoever becomes vain. WTien such a point of view obtains the most 

 that can be attained is a quarrel of words, devoid of real meaning. 

 The matter ends in an impasse, and nothing remains but to close the 

 discussion, franldy admitting that all knowledge of the nature of a 

 being is impossible. This is said, however, without assuming that any 

 student will arise to champion such an absurdity. 



It remains to us, therefore, to attempt to determine whether the 

 bacteriophage corpuscle does or does not possess this combination of 

 properties which constitutes the criterion of life. 



* "But" it may be said, "this is not quite true, we say that it is living because 

 it multiplies." This is not correct. Unconsciously we reason differently. 

 Immerse a crystal of copper sulfate in a saturated solution of this salt. The 

 crystal multiplies. Do we infer that the crystal is living? No. Immerse a 

 crystal of the same kind in a saturated solution of iron sulfate. There is no 

 multiplication. Do we say that it is dead? Obviously not. It is then, not because 

 the bacterium multiplies that we consider it a living being, but because it multi- 

 plies by using the heterologous substances of the nutritive medium, transforming 

 them into its own distinctive substance. We say it is living because it assimilates. 



