52 ESSENTIALS OF BIOLOGY 



i^d. Excess- Values in Animal Structure. Regarded as 

 mechanisms, animal structures need not exceed a certain degree 

 of complexity. But in colours, patterns or pigment, forms and 

 sculpturings on shells, frustules and other external hard parts, 

 feathering, plumes and crests, fantastic bodily forms, etc., we seem 

 to see what we, as artisans, should call " ornament " and over- 

 elaboration : structural detail that is, so far as we can find by 

 experiment and obser\'ation, unnecessary for the effect that the 

 bodily part or organ may produce in order that the general 

 behaviour of the animal may be subserved. This is " excess- 

 value." 



15. ON ANIMAL STRUCTURE AND ITS SIGNIFICANCE 



IN GENERAL BIOLOGY 



15a. Structural Mechanisms. We may here regard a 

 mechanism as a system of parts that are placed in relation to each 

 other in a definite way. When these parts are actuated a certain 

 definite effect follows and the nature of this effect depends on the 

 ways that the parts of the machine are placed in relation to each 

 other. 



Thus the human arm is such a system of parts. Shoulder- 

 blade and humerus are so articulated that the latter moves in a 

 ball and socket joint. The forearm bone (the ulna) is so 

 articulated with the humerus that a hinge joint is formed : thus 

 the natures of the movements of humerus and forearm that are 

 possible are determined by the natures of these articulations. 

 Muscles are attached between the various bones in such ways 

 that their tensions apply forces to those bones. Finally, the 

 muscles are energized. Essentially we have systems of levers, 

 in the mechanical sense. 



All the grosser mechanisms of the animal body can be described 

 in analogous ways. 



In the earlier conceptions of the animal body as a machine it 

 was such mechanical (in the classic sense) conceptions that w^re 

 made. But later speculations extended the notion of parts of 

 a mechanism to physical systems containing tubes of varying 

 calibre through which liquids flowed ; filters with pores of varying 

 size ; liquids, quasi-liquids and gases (the " spirits ") that were 

 expanded or contracted when the temperature varied and so on. 



Thus, what we call, in ordinary language, physical structure 



