304 Problems of Organic Adaptation 



The mechanical principles of the wheel and the screw are 

 not found anywhere among organisms for the obvious 

 reason that every portion of the animal machine must be 

 connected by blood vessels and nerves with the central part, 

 but the use by animals of levers of all kinds and for every 

 conceivable purpose has never been surpassed in machines 

 of human invention. 



The motive power of the living machine is found in pro- 

 toplasmic contractility whether it manifests itself in amoe- 

 boid, ciliary, or muscular movement. But since all movement 

 of large bodies must be brought about by muscles, we may 

 limit our consideration to this type of movement. In spite 

 of the fact that more attention has probably been devoted 

 to the structure and function of muscle than to any other 

 animal tissue, the ultimate causes of muscular contraction 

 are still problematical. Nothing comparable to this form 

 of motion exists except among animals. It is known that 

 the chief source of chemical energy in muscular contrac- 

 tility is the burning of dextrose, but the manner in which this 

 chemical energy is transformed into mechanical energy is 

 unknown. However, the relative efficiency of different types 

 of engines is known, and they may fairly well be compared 

 with the living engine. The ordinary steam engine trans- 

 forms about 10% of the energy of the steam into motion; 

 the steam turbine, about 17%; the Diessel internal combus- 

 tion engine has a practical efficiency of about 30%, while 

 muscle has a net efficiency of from 20% to 30%. The living 

 engine is therefore more efficient than the steam engine and 

 about as efficient as the best type of engine that has been de- 

 vised by man. But in addition to this, the temperature 

 developed in muscle is much less, and the flexibility of the liv- 

 ing machine, as measured by the rate or extent of movement, 

 is much greater than in any other engine. For example, the 



