154 THE INDUCTIONS OP BIOLOGY. 



the body is composed. In the flesh of adult mammalia, water 

 forms from 68 to 71 per cent., organic substance from 24 to 

 28 per cent., 'and inorganic substance from 3 to 5 per cent. ; 

 whereas in the foetal state, the water amounts to 87 per cent., 

 and the solid organic constituents to only 11 per cent. Clearly 

 this change from a state in which the force-evolving matter 

 forms one-tenth of the whole, to a state in which it forms two 

 and a half tenths, must greatly interfere with the parallelism 

 between the actual and the theoretical progression. Yet 



another difficulty may come under notice. The crocodile 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 organization 

 have no definite limits of growth, is, however, an exceptional 

 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 maintenance of heat. 

 It is habitually inert: not usually 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 ex- 

 pends in muscular action an amount of force that is insignifi- 

 cant 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. To over- 

 come gravitative force the creature has not to expend a 



