THE MEANING OF EVOLUTION 277 



plant organism is essentially a system in which energy 

 is accumulated in the potential form. Then, in the 

 animal kingdom we see that the main tendency of 

 evolution has been the development of systems in 

 which energy becomes expended in infinitely varied 

 movements. It may seem, on superficial examination, 

 that in the animal mode of metabolism energy is dis- 

 sipated as it is in inorganic processes ; and this is the 

 conclusion that we should reach if we considered the 

 actions, and the results of the actions, of the lower 

 animals only, that is, animals lower than man. We 

 return to this point later on, but in the meantime it is 

 to be noted that the fundamental division of organisms 

 is that founded upon their activities as energy-trans- 

 formers, that is, into plants and animals. Within each 

 of these kingdoms of organisms structural evolution 

 has occurred : the unicellular green plant has evolved 

 along very numerous lines, each of them characterised 

 by a different type of morphological structure. The 

 unicellular animal has also evolved in a similar way 

 with the result that the present phyla have become 

 established. Looking at these great groups of animals, 

 we see that two of them have attained dominance by 

 the development along different lines of a sensori- 

 motor system. Here we see another fundamental 

 difference between the plant and animal organism, but 

 one which is a consequence of the difference that exists 

 between the two kingdoms in respect of the energy- 

 transformations carried out by them. The plant is 

 characterised by immobility, the animal by mobility. 



Immobility implies unconsciousness, mobility con- 

 sciousness, and this physical difference is the third one 

 which we can establish between the plant and the 

 animal. Now few physiologists are likely to accept 

 this distinction as one which has any real objective 



