22 ESSAYS OF A BIOLOGIST 



unless put into relation with proper effector organs — 

 organs for action, whether locomotor or secretory. 

 So that the first steps are the elaboration of sense-organs, 

 the increase of efficiency of muscles and glands, and, 

 equally essential, the construction of an improved 

 ' adjuster system^ whereby the stimulus falling on 

 the sense-organ may be translated into action and into 

 the right kind of action. This adjustor mechanism 

 is the central nervous system. Most of the further 

 history of organisms may be summed up in one phrase 

 — the evolution of adjustor mechanisms. 



At first, it is chiefly of importance to be brought 

 into relation with more and more of the happenings 

 of the outer world, to be able to see and hear and feel 

 and smell more and more delicately ; and to react 

 upon the outer world more and more efficiently and 

 powerfully, to be able to move and to handle matter 

 more quickly and with finer and finer adjustment. 



But unless the adjustor mechanism be improved, 

 this process soon tends to a limit. I may illustrate 

 my meaning by a simple supposition. Suppose an 

 organism capable of very little beyond reflexes and 

 instincts and with but a scanty dose of associative 

 power : of what conceivable use to it would be a 

 telescope or a telephone ? Man obtains a biological 

 advantage from such accessory sense-organs in that, 

 when thus apprised of events at a distance, he is enabled 

 to plan out courses of action to meet the events which 

 he imagines are going to overtake him : but both 



