26 MIND IN EVOLUTION CHAP. 



It is merely a familiar instance of the same well adapted 

 rhythm, that work makes us hungry, that being hungry 

 we find something to eat, that eating it makes us able to 

 work again, and thereby earn our next meal. It is a more 

 special application of the same fact that the muscle which 

 we especially exercise gets an extra supply of blood and 

 grows in proportion. 



3. Thus both in general and in special ways the normal 

 healthy life is a process of unceasing oscillation about an 

 imaginary equilibrium point, departure from which tends 

 of itself to set up processes which take the organism back 

 again. The same feature of organic action appears in a 

 more curious shape in more or less abnormal life. A 

 machine can be made to regulate its own action within 

 certain limits. Thus, in the steam engine, the forward 

 thrust of the piston opens and shuts valves by which the 

 backward thrust is at once brought about. Here there is 

 perhaps a parallel to the automatic rhythm of breathing, or 

 of the heart's beat. More than that, by the device of the 

 " governor " a steam engine can regulate its own available 

 energy in accordance with the work required of it. There 

 is a close analogy here to the laboured breathing of hard 

 exercise where more oxygen is required. But I do not 

 think that any machine can repair itself when its own 

 structure is in some way injured. This, within limits, 

 [every organism can do. Even the human body can repair 

 its skin and its broken bones. This repairing means that 

 in the healthy organism a lesion sets up processes which 

 tend to restore the normal condition. Let us first observe 

 the degree and kind of adaptation involved in the healing 

 of a broken bone. The spongy substance of the long 

 bones in the higher vertebrates is 



" . . . . arranged on a similar mechanical principle to that ot 

 arched structures in general ; it is composed of numerous fine bony 

 plates, so arranged as to withstand the greatest amount of tension 

 and pressure, and to give the utmost firmness with a minimum 

 expenditure of material. But the direction, position, and strength 

 of these bony plates are by no means congenital or determined in 

 advance : they depend on circumstances. If the bone is broken 

 and heals out of the straight, the plates of the spongy tissue 



