22 ESSAYS OF A BIOLOGIST 



gans — organs for action, whether locomotor or secre- 

 tory. 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 ac- 

 tion 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 mecha- 

 nisms. 



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 en- 

 abled to plan out courses of action to meet the events 

 which he imagines are going to overtake him: but 

 both planning and imagination are entirely functions 



