4 THE MECHANISM OF LIFE 



An animal is 



Something that tends to preserve its own individual exist- 

 ence; 



that nourishes itself; and 

 that reproduces itself. 



Self-preservation, nutrition, and reproduction are, therefore, 

 the characteristics or the essential marks of life. We may call 

 them the primary instincts, tendencies, or impulses of animality. 



If we knew only the structure of animals, we might be able to 

 find what were the functions or things done by those structures, 

 but the history of comparative anatomy gives us many instances 

 of our failure to do this. And, conversely, a detailed knowledge 

 of the things that an animal can do does not always enable us 

 to discover the kind of structure or mechanism that is at work. 

 In a sense it is all wrong to speak about an animal as a structure, 

 or even as a thing. It is " something happening." 



And yet we are about to write several chapters concerning the 

 " animal mechanism," " organs," " parts," etc., and so we must 

 point out that it is not in any spirit of paradox that we take the 

 dynamic view of animality suggested above. " If," said Lord 

 Kelvin, " I can make a mechanical model, I comprehend." This 

 is our way of investigating, not only life, but everything of which 

 we can become cognisant. The human mind is essentially con- 

 structive, and it tries to act upon everything that is outside 

 itself; that the animal must act in this way is the reason that 

 there is a mind at all. When we look at a mechanism and try to 

 understand how it works, we either actually take it to pieces or 

 we do so in imagination we make an analysis of an activity of 

 some kind. We cannot describe a marine engine without con- 

 sidering the cylinders, cranks, crank shaft, reversing gear, etc., 

 even though we know quite well that all these parts exist, as 

 parts, only in the case of an engine that does not go. When we 

 think of, or see, the mechanism at work there are no parts, and, 

 of course, there are none after the engineer " assembles " them. 

 And so our conception of the organs and parts of the animal body 

 is only our analysis of the means of life. In a way they are what 

 Lord Kelvin called his mechanical models, and their usefulness 

 is that they enable us to comprehend and investigate. But in 

 the normal, living animal the parts are integrated, and their 

 activity is an indivisible one. 



