126 THE INDUCTIONS OF BIOLOGY. 



is said to grow as long as it lives ; and there appears reason 

 to think that some predaceous fishes, such as the pike, do 

 the same. That these animals of comparatively high organ- 

 ization, have no definite limits of growth, is, however, an ex- 

 ceptional fact due to the exceptional non-fulfilment of those 

 conditions which entail limitation. What kind of life does 

 a crocodile lead? It is a cold-blooded, or almost cold- 

 blooded, creature ; that is, it expends very little for the main- 

 tenance of heat. It is habitually inert: not chasing prey, but 

 lying in wait for it ; and undergoes considerable exertion 

 only during its occasional brief contests with prey. Such 

 other exertion as is, at intervals, needful for moving from 

 place to place, is rendered small by the small difference 

 between the animal's specific gravity and that of water. 

 Thus the crocodile expends in muscular action, an amount of 

 force that is insignificant compared with the force commonly 

 expended by land-animals. Hence its habitual assimilation 

 is diminished much less than usual by habitual waste ; and 

 beginning with an excessive disproportion between the two, 

 it is quite possible for the one never quite to lose its advance 

 over the other while life continues. On looking closer into 

 such cases as this and that of the pike, which is similarly 

 cold-blooded, similarly lies in wait, and is similarly able to 

 obtain larger and larger kinds of prey as it increases in size ; 

 we discover a further reason for this absence of a definite 

 limit. The mechanical causes necessitating a limit, are here 

 only partially in action. For a creature living in a medium 

 of nearly the same density as its body, has not constantly to 

 overcome that gravitative force which is the chief resistance 

 to be met by terrestrial animals : it has not to expend for 

 this purpose, a muscular power that is large at the outset, and 

 increases as the cubes of its dimensions. The only force in- 

 creasing as the cubes of its dimensions, which it has thus to 

 overcome, is the inertia of its parts. The exceptional con- 

 tinuance of growth observed in creatures so circumstanced, is 

 therefore perfectly explicable. 



