320 THE BACTERIOPHAGE AND ITS BEHAVIOR 



rence. And its importance is very considerable, for symbiosis is in large 

 measure responsible for evolution.* The term symbiosis is, however, 

 rather poorly chosen, for it dates from an epoch when it was thought 

 that the communal existence of the two beings necessarily involved a 

 mutual benefit. We now know that symbiosis is always a struggle, 

 which, as a rule, ends in the elimination of one of the two contestants. 



But the history of science presents a very curious thing in the discus- 

 sions which crop up each time that a new example of symbiosis is dis- 

 covered. The last one, prior to the one now engaging our attention, 

 related to the lichens. It is unwise to go into details which would 

 carry us too far afield, although the alga-fungus symbiosis, constitut- 

 ing a lichen, is in all respects comparable to that between the bacterium 

 and the bacteriophage It is sufficient to recall that when Schwendener 

 had shown that the being which was thought to be an individual was in 

 reality dual in nature, there were impassioned discussions, and Nylan- 

 der, the most noted lichenologist of the day, refused up to his death to 

 accept such a manifest heresy. 



A mixed culture, comprising bacterium and bacteriophage, is in 

 reality a culture of a microscopic lichen. A bacterium in symbiosis 

 with a bacteriophage is a "microlichen." And it would not be strange 

 (I say this advisedly, for, although as yet incontrovertible evidence is 

 lacking, I have reason to suspect such a thing) if certain bacterial 

 species may be recognized only when in the form of "microlichens," 

 that is, as mixed cultures. 



We have seen from our study of mixed cultures that it is experimen- 

 tally easy to transform a normal culture into a "lysogenic" culture, the 

 only essential being to transfer, en bloc, a secondary culture. Can such 

 strains be purified? As we know it is only necessary to transfer them 

 through individual isolated colonies and the majority become ultrapure. 



Nevertheless, if we permit a secondary culture to age for several 

 months, we find that the ''bacteriophage-bacterium" symbiosis becomes 

 more and more intimate. At the outset, if 20 colonies are isolated 

 from such a secondary culture, one only (sometimes none) may be 

 mixed, the other 19 being ultrapure. With an old mixed culture, the 

 percentages are reversed, the mixed colonies greatly outnumber the 

 ultrapure colonies. 



For two years I have maintained .two mixed cultures (derived from 



* The study, as yet hardly touched upon, of the mutations which take place 

 in the course of the Bacteriophage-Bacterium symbiosis, is of a nature to throw 

 much light upon the phenomena of evolution. 



