50 MIND IN EVOLUTION CHAP. 



the contraction, the tension of the muscular fibre increases the 

 facility with which the explosive changes resulting in a con- 

 traction take place. And we have other evidence that anything 

 which tends to stretch the muscular fibres, whether during rest or 

 during contraction, increases the metabolism of the muscle." l 



Here again physiologists offer a mechanical explanation 

 of something which looks like a special effort; We 

 have not in this place to pass on these explanations in 

 particular instances. It is enough for us that any action 

 may be considered reflex as long as it is (i) a response 

 of uniform type to present sensory stimulus, or (2) a 

 resultant of tendencies to such uniform response acting 

 together. Action that can be so explained is mechanical. 

 That which cannot be so analysed is conational. 



8. To put together our results up to this point. We 

 have found two classes of actions which proceed uniformly 

 from pre-existent structure and are not determined by 

 relation to the results accruing from them in the particular 

 instance. The first class are initiated by processes within 

 the organism, the second by stimulus from without. The 

 latter we call Reflex action, which we define as the action 

 of a structure which is specially framed so as to respond 

 to present sensory stimulus in a uniform manner. Such 

 structures may be formed under the influence of natural 

 selection, 2 where it is in general beneficial to the organism 

 to respond in one way rather than another to stimuli of 

 any given kind. The reflex action is not called forth or 

 determined by the purpose that it serves in the economy 

 of the organism, for it persists even where useless or 

 injurious. It is, however, subject in varying degrees to 

 the condition of the organism as a whole, and thereby 

 arises a further adaptation of its action to what is required. 

 In the typical reflex this subjection is at a minimum, and 

 there is uniform response of a special structure to appro- 

 priate stimulus. The more absolute this uniformity, the 

 more close does the reflex approach the dead-mechanical 

 type. But the response of the special structure may, in 



1 Foster, I. p. 143. 



2 They may also be formed at least in higher organisms by habit, and 

 when so formed have the same general character. 



