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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 planning and imagination are entirely functions of an adjustor mechanism, and without such a mechanism, great enlargement of sensory power would only result in an organism reacting too often and unnecessarily to events in its environment.