BIOLOGICAL ORDER 



Table VIII. Genes and Viruses. 



material when it is in the form of a provirus. The expressions of 

 the viral functions are then blocked, that is, repressed, and the 

 lysogenic bacterium grows and multiplies. But the prophage is 

 present, a self-perpetuating sword of Damocles. When it is dere- 

 pressed, the functions for its autonomous reproduction are set free: 

 the vegetative phase is initiated, and the bacterium is killed. 



As a consequence of the presence of the prophage, the lysogenic 

 bacterium necessarily differs from the nonlysogenic one. The fact 

 that lysogenic bacteria are frequently found in nature seems to 

 indicate that the prophage might sometimes be useful, partly perhaps 

 because it confers immunity on the bacterium. But the bacterium 

 is an independent unit. A provirus may alter its competitive value 

 and thus modify the outcome of a competition, but the bacterium/ 

 provirus system, considered in itself, is perfectly viable. This should 

 be true also for an animal cell/v4rus system. However, the bacterium 

 is an organism, an independent system, whereas the animal cell is 

 a dependent part of an organism. 



Some animal viruses generally kill the cell they infect. This is the 

 case for the agents of acute diseases such as poliomyelitis, smallpox, 

 and yellow fever. These viruses are comparable to the virulent 

 phages. Other animal viruses behave like the temperate phages. 

 This is true especially of oncogenic viruses, the viruses responsible 



[84] 



