NUTRITION OF THE ANIMAL. 99 



environment be not too sudden or extreme. In other words, 

 the organism possesses a certain plasticity which enables it to 

 adapt itself to gradually -changing conditions of the environment. 

 Now there is good reason to believe that as environment 

 has gradually undergone changes in the past, organisms have 

 gradually undergone corresponding changes of structure. Those 

 which have become in any way so modified as to be most per- 

 fectly adapted to the changed environment have tended to sur- 

 vive and leave similarly -adapted descendants. Those which 

 have been less perfectly adapted have tended to die out through 

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

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

 of the Fittest' -the remarkable adaptations everywhere met 

 with 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 first 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 bodv, like anv other machine, while life con- 



/ . i/ 



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

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

 energy expended in vital action is the working protoplasm itself, 

 which undergoes a destructive chemical change (katabolism or 

 destructive metabolism) having the nature of an oxidation. From 

 this it follows on the one han 1 that the waste products of this 

 action must be ultimately passed out of the body as excretions, 

 and on the other hand that the loss must ultimately be made 

 good by fresh supplies entering the animal in the form of food. 

 It is further evident that the income must equal the outgo if the 

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

 mal is to grow. 



