414 LAWS OF MULTIPLICATION. 



in the inner actions required to counter-balance tliera. Even 

 if species were similarly conditioned, self-preservation would 

 require of them extremely unlike expenditures of force. 



The cost of locomotion increases in a greater ratio than the 

 size. In virtue of the law that the weights of animals increase 

 as the cubes of their dimensions, while their strengths increase 

 onty as the squares of their dimensions (§ 46), a given speed 

 requires a large animal to consume more substance in propor- 

 tion to its weight, than it requires a small animal to consume ; 

 and this law holding of all the mechanical actions, there 

 results, other things equal, a difficulty of self- maintenance 

 that augments in a more rapid ratio than the bulk. Nor 

 must we overlook the further complication, that among 

 aquatic creatures the variation of resistance of the medium 

 partially neutralizes this effect. 



Again, the heat-consumption is a changing element in the 

 total expense of self-preservation. Creatures that have tem- 

 peratures scarcely above that of the air or water, may, other 

 things equal, accumulate more surplus nutriment than 

 creatures that have to keep their bodies warm spite of the 

 continual loss by radiation and conduction. This difference 

 of cost is modified by the presence or absence of natural 

 clothing ; and it is also modified by unlikenesses of size. Here 

 the bulky animals have the advantage : small masses cool- 

 ing more rapidly than large ones. 



Dissimilarities of attack and defence are also causes of 

 variation in the outlay for self-maintenance. A creature 

 that has to hunt, as compared with another that gets a 

 sufficiency of prey by lying in wait, or a creature that 

 escapes by speed as compared with another that escapes by 

 concealment, obviously leads a life that is physiologically 

 more costly. Animals that protect themselves passively, 

 as the Hedge-hog by its spines or as the Skunk and the 

 Musk-rat by their intolerable odours, are relatively econo- 

 mical ; and have the more vital capital for other purposes. 

 Amplification is needless. These instances will show that 



