INTRODUCTION / 



descendants of some very primitive form of life that originated 

 prior to the evolution of organized cells (Oparin, 1938). This 

 hypothesis is not very useful or satisfying, but it could be true for 

 some or all phages, or for soine or all viruses. The degeneration 

 hypothesis considers that viruses have gradually evolved from 

 more complicated forms of life by the loss of all protoplasm un- 

 necessary to the peculiar mode of existence bacteriophages have 

 chosen. This hypothesis has a certain intellectual appeal in that 

 it is easy to visualize intermediate steps in the degenerative proc- 

 ess, and progressive loss by bacteria of synthetic abilities is sub- 

 ject to experimental study. This hypothesis avoids the problem 

 of the ultimate origin of life, which is not necessarily related to 

 the problem of the origin of viruses, and it provides for the in- 

 dependent evolution of different viruses. By this hypothesis 

 there is no need to assume that animal viruses and bacterio- 

 phages are derived from a common progenitor. 



Recent studies of the phenomenon of lysogeny have lent 

 credence to a variant of the degeneration hypothesis that maV be 

 called the "mutation" hypothesis (Lwoff, 1953). According to 

 this hypothesis a series of mutations occurring in a segment of the 

 genetic material of a bacterium has conferred on that segment 

 a certain degree of autonomy. The mutations resulted in the 

 synthesis of a specific kind of protein covering to protect the seg- 

 ment of genetic material from damage by the extracellular en- 

 vironment and to facilitate the transfer of the genetic material 

 from one cell to another. It is not impossible that bacterio- 

 phages may have evolved from a primitive mechanism of 

 sexuality originally developed for the purpose of transfer of ge- 

 netic materials between bacterial cells. At least certain bacterio- 

 phages are able to perform that function at present. According 

 to this hypothesis the phage genetic material would be a modi- 

 fied form of the bacterial genetic material, yet sufficiently like 

 the homologous segment of bacterial gene stuff to undergo ge- 

 netic recombination with it or even to replace it. This hypothesis 

 is consistent with the observed fact that the genetic material of 

 certain temperate phages can associate more or less perma- 



