40 MIND IN EVOLUTION CHAP. 



" pseudopodium " in one direction, or draw in another 

 from an opposite quarter. If the nature of the molecular 

 change is to produce a large mass of unstable chemical 

 compounds in a certain part, then the effect of the final 

 touch will be to " explode " this mass, and a sudden 

 and violent contraction follows. In the higher organisms, 

 besides the interaction of molecules in each cell, there is 

 the interaction of many distinct cells, and indeed distinct 

 organs, to be taken into account. The restless move- 

 ments of the infant may be due to changes in the nerve 

 centres, but they may also arise from a thousand and one 

 different stimuli from the muscles, joints, digestive organs, 

 or any part of the body, and indeed of other things with 

 which the body comes into contact. It thus becomes 

 impossible to draw a clear and certain line between actions 

 of this class and the lowest kind of reflexes, with which 

 we shall have presently to deal. It remains only to add 

 that in the case of automatic actions the internal changes 

 of which we speak appear to have become regulated, pre- 

 sumably under the influence of natural selection, so as to 

 follow one another in a fixed order and at a determinate 

 speed. There are the same processes of building up and 

 breaking down, and they give rise in the same way to 

 contraction and expansion, but they have become more 

 accurately adjusted to one another in their proportions, so 

 that each contraction is of measured length, force, and 

 speed, and is succeeded at a fixed interval by an equally 

 determinate movement of expansion. 



2. Reflex Action. 



Hitherto we have dealt with actions resulting from 

 inward changes. We have now to consider actions 

 brought about by the influences of the environment. 

 All organisms may be affected by the direct contact of 

 outer things with their surface, and most organisms, except 

 the lowest, can be affected in other ways as well as 

 through their organs of sense. Such an affection is called 

 a stimulus, and the resulting change in the organism a 

 rejjrjpjise. It is of course principally by its responses to 

 stimuli that the organism succeeds in adjusting its 

 behaviour towards changes in the outer world in a way 



