14 LIFE IN NATURE. 



but is conformable to all that we best know and 

 most easily understand. The same principles are 

 acted upon by every boy who makes a bird-trap with 

 tiles and a few pieces of stick : here is the opposition 

 to gravity, the equilibrium of force and resistance, 

 and the unfortunate bird applies the stimulus. 



But if the case be so simple, why has it not always 

 been presented so ? Why has it been conceived that 

 the living body had an inherent activity peculiar to 

 itself? And why especially has the decomposition 

 of the body been represented as the result, and not 

 as the cause, of its activity ? Many circumstances 

 have contributed to make this problem difficult of 

 solution. In the first place, if the animal is like 

 a machine in some respects, in others it is strikingly 

 unlike one. All machines consist of two distinct 

 parts : the mechanism and the power. First, men 

 construct the boiler, the cylinder, the levers, the 

 wheels, all the parts and members of the steam- 

 engine, and then they add the water and the fire 

 first, they arrange the wheels, the balances, the 

 adjustments of the watch, and then they bend the 

 spring. In the body these two elements are united, 



