THE ANIMAL ACTION-SYSTEMS 69 



that direction in which entropy increases. It is not possible 

 so to bring about a universal physical change in which entropy 

 decreases. We can observe that water flows, of itself, from higher 

 to lower levels (dissipating energy by friction as it does so). We 

 can cause water to flow from lower to higher levels, by pumping 

 it through pipes against a head of water, but in such a case we 

 set up a coupled transformation, connecting the natural physical 

 change with an artificial one. Then the gain in potential energy 

 obtained by raising the water to a higher level is less than the 

 quantity of energy dissipated in the pumping. Or we can cause 

 heat to flow from the atmosphere of a cold-storage room to a 

 reservoir of compressed air by the operations of compression ; or 

 we can make heat flow from cold compressed ammonia gas (in 

 a refrigeration plant) into the warmer atmosphere. But in such 

 cases extraneous energy is again degraded. 



So all energy-transformations occur in one way (ultimately) — 

 in such a way that universal available energy is (from our point 

 of view) expended irretrievably. 



//. THE ANIMAL ACTION-SYSTEMS 



According to the structural bodily plans that have been evolved 

 so the action-systems differ : Thus we have the familiar pedal 

 locomotory actions of, say, the mammals, the creeping locomotions 

 of the snail, the ciliary movements of infusoria, etc. 



23. ON PEDAL LOCOMOTION AND ASSOCIATED ACTIONS 



Bi-Pedal locomotio7i is exhibited by man and some other Primate 

 mammals and by birds. The motile appendages are limbs 

 articulated with the body and jointed in themselves, thus the 

 human leg with thigh, shin, ankle and pedal phalanges, all 

 articulating with each other and provided with appropriate 

 musculatures so that the limb has freedom of motion in all three 

 dimensions of space. The terminal parts of the limb can exercise 

 gripping action on the ground. The limb moves mainly as a 

 pendulum, its pivot being the articulation with the pelvis and 

 rapidity of movement is dependent, to some extent, upon the 

 pendular length. Varying methods of articulation, and varying 

 fusions of the skeletal parts (as in the *' ankle " parts of the bird 

 leg) modify the freedom of movement of the whole limb ; thus 



