SIMPLE AND COMPOUND EVOLUTION. 311 



quantities of contained motion are accompanied by differ- 

 ences in the amounts of re-distribution. 



The contrasts among organisms in chemical composition 

 yield us the first illustration. Animals are distinguished 

 from plants by their far greater amounts of structure, as 

 well as by the far greater rapidity with which changes of 

 structure go on in them; and in comparison with plants, 

 animals are at the same time conspicuous for containing im- 

 mensely larger portions of those highly-compounded nitro- 

 genous molecules in which so much motion is locked up. 

 So, too, is it with the contrasts between the different parts of 

 each animal. Though certain nitrogenous parts, as carti- 

 lage, are inert, yet the parts in which the secondary re-dis- 

 tributions have gone on, and are ever going on, most active- 

 ly, are those in which the most highly-compounded mole- 

 cules predominate ; and parts which, like the deposits of -fat, 

 consist of relatively-simple molecules, are seats of but little 

 structure and but little change. 



We find clear proof, too, that the continuance of the 

 secondary re-distributions by which organic aggregates are 

 so remarkably distinguished, depends on the presence of 

 that motion contained in the water diffused through them ; 

 and that, other things equal, there is a direct relation be- 

 tween the amount of re-distribution and the amount of 

 contained water. The evidences may be put in three 

 groups. There is the familiar fact that a plant 



has its formative changes arrested by cutting off the supply 

 of water: the primary re-distribution continues it withers 

 and shrinks or becomes more integrated but the secondary 

 re-distributions cease. There is the less familiar, but no 

 less certain, fact, that the like result occurs in animals oc- 

 curs, indeed, as might be expected, after a relatively smaller 

 diminution of water. Certain of the lower animals furnish 

 additional proofs. The Rotifera may be rendered apparent- 

 ly lifeless by desiccation, and will yet revive if wetted. 

 "When the African rivers which it inhabits are dried up, the 



