NUTRITION OF THE AMMAL. 99 



environment be not too sudden or extrenic In utlicr words 

 the organism possesses a certain i^lastlmtij wliicli enal)let^ it to 

 adapt itself to graduallj-clianging conditions of the environment. 

 Now tliere is good reason to believe tliat as envirunmunt 

 has gradually undergone changes in tlie })ast, organisms have 

 gradually undergone corresponding clianges of structure. Those 

 which have become in any way so modilied as tu he most per- 

 fectly adapted to the changed environment have tended to sur- 

 vive and leave similarly-adapted descendants. Those wliich 

 have been less j)erfectlj adapted have tended to die out tlirough 

 lack of fitness for the environment ; and by this process — called 

 bj Darwin '' Natural Selection" and by Spencer the "Survival 

 of the Fittest" — the remarkable adaptations everywhere met 

 wdth are believed to have been gradually worked out. 



It should be observed that Natural Selection does not really explain the 

 origin of adaptations, but only their persistence and accumulation. The 

 theory of evolution is not at present such as to enable us to say with cer- 

 tainty what causes the tirst origin of adaptive variations. 



Nutrition. The earthworm does work. It works in travel- 

 ling about and in forcing its way through the soil ; in seizing, 

 swallowing, digesting, and absorbing food ; in pumping the 

 blood; in maintaining the action of cilia; in receiving and send- 

 ing out nerve-impulses; in growing; in reproducing itself — in 

 short, in carrying on any and every form of vital action. To 

 live is to work. Now work involves the expenditure of energy, 

 and the animal body, like any other machine, while life con- 

 tinues, requires a continual supply of energy. It is clear from 

 what has been said on p. 32 that the innnediate source of tho 

 energy expended in vital action is the working protoplasm itsolf, 

 which undergoes a destructive chemical change (kataholism or 

 destructive metabolism) having the nature of an oxidation. I'^i-nni 

 this it follows on the one hanl that the waste ])ro(lucts of this 

 action must be ultimately passed out of the body ;us excretions, 

 and on the other hand that the loss must ukimately ]>e made 



good by fresh supplies entering the animal in the form of f 1. 



It is further evident that the income must ecpnU the (Uitgo if tiie 

 animal is merely to hold its own, and must exceed it if the ani- 

 mal is to grow. 



