138 FRANCIS D. CARLSON 



detect the presence of predators or sense a deleterious environment, 

 although it is certainly true that some can. Even so, the speeds with 

 which they travel, tens to hundreds of microns per second, are hardly 

 enough to get them away from some of the faster swimming micro- 

 organisms. In fact, in the case of bacteria which are subject to preda- 

 tion by bacteriophage, a predator that probably does not sense its 

 prey until it collides with it, it is clear that in the presence of a horde 

 of phage particles the execution of a random, helter-skelter escape 

 pattern on the part of the bacterium will only increase the chances 

 of its being caught because such a motion increases the probability 

 of collision with a phage particle. In the presence of phage the bac- 

 terium is less likely to get caught if it remains completely motionless. 

 Still another reason for not considering these arguments is the fact 

 that they do not readily lend themselves to a formal analysis of the 

 type that can be expected to lead to conclusive results. True, it is 

 possible to make appropriate simplifying assumptions but in so do- 

 ing one is very likely to weaken the analysis to such an extent as to 

 render it of little real significance. 



On the other hand, the need to collect nutrient substances from the 

 surroundings in order to survive is common to all forms. The viruses 

 constitute a possible exception but even in this instance growth, if 

 not survival, requires the presence of living cells. On the basis of 

 generality, therefore, the argument that motility increases the ca- 

 pacity for the organism to collect food is the only one that is not 

 readily ruled out. While it is obvious that motility increases the 

 capacity to collect food, it is not obvious that at the same time it 

 always, or even ever, results in a net increase in the energy stores of 

 the organism; for there can be no doubt that the process of moving 

 itself requires energy, and unless there is a net gain in energy above 

 the minimum required for existence, there can be no net growth, and 

 indeed the organism may eventually commit suicide by continuing 

 to move. From the fact that motile forms do exist we can conclude of 

 course that there are conditions under which it is possible to collect 

 more energy by moving than the process of moving requires. This 

 does not, however, tell us what the conditions are. We seek here to 

 develop these conditions. 



