one might suppose, for they eertainly can't match the richness of 

 our culture and technology. But evolution is only marginally inter- 

 ested in such issues; they are important only insofar as they affect 

 an animal's ability to survive until the age of reproduction. So what 

 would an elephant have to remember? If one has never thought about 

 that question, the answer may come as a surprise. An elephant has 

 to remember no less and no more than every other creature that ever 

 lived. For the fact is that all creatures, great and small, have only 

 ever had to remember one thing: how to move under the prevailing 

 circumstances in the environment and within their own bodies. On 

 the output side, therefore, all that an animal can ever do is move. 



Note that I use the term on the output side, rather than response, 

 which might seem more appropriate. I chose the former because I 

 wanted to include reactions to the conditions within an animal's 

 body, as well as reactions to external stimuli. And just as impor- 

 tantly. I avoided making any reference to nervous systems, because 

 I wanted to include animals not possessing such things. 



Let us consider one such lowly organism, namely the single- 

 celled Escherichia coli bacterium that does yeoman service in our 

 digestive system. This creature is important to the argument because 

 it does not respond rapidly to external stimuli; it has no reflexes. 

 When a coli bacterium swims in water, it meets as much resistance 

 as we would when swimming in molasses. But it has to swim, in 

 order to reach its food. There is a second surprise, however, because 

 the bacterium has no senses; it does not know where potential food 

 is located. Why, then, would it swim? The short answer is that it 

 evolved to do so, because that proved to be a useful survival strat- 

 egy. Let's take a look at that strategy, drawing on the brilliant work 

 of Howard Berg (1993) as we go. 



The bacterium is pushed through the water by its flagellum, which 

 resembles a ship's propeller. Anti-clockwise rotation of that device, 

 looking in the forward direction, produces forward motion. Clock- 

 wise rotation does not lead to backward travel, as might be expected; 

 it causes the creature to tumble and change direction. The bacteri- 

 um's movement is dictated by the conditions within its body — by 

 what in a more advanced creature would be called its drive. The 

 flagellum's rotation, clockwise or anti-clockwise, is specified by the 



