270 THE PHILOSOPHY OF BIOLOGY 



movements of its parts, they are few in number, they 

 belong to a few simple types, and they are evoked by 

 simple external physical changes in the medium. The 

 movements of the typical animal are movements of 

 the organism as a whole ; they are infinitely varied in 

 their nature ; they are evoked by individualised stimuli 

 and they are continually being modified by the experi- 

 ence of the organism. 



The bodily structure of the animal is the means 

 whereby this purposeful mobility is attained and the 

 energy-transformations directed ; and the greater and 

 more varied the movements of the animal, the more 

 complex is its structure. In respect of the manner in 

 which the energy-transformations are effected, that is, 

 in respect of the material means whereby energy falls 

 from a state of high potential to a state of low potential, 

 the morphology of the animal is similar to that of the 

 plant, that is, the energy-transformations are the 

 functions of nucleated cells. But in the plant the 

 kinetic energy of solar radiation passes into the potential 

 energy of chemical compounds which become stored 

 in the body of the plant ; while in the animal the 

 potential energy of ingested chemical compounds 

 passes into the kinetic energy of the movements of the 

 animal itself. How exactly it moves, how this kinetic 

 energy is employed is determined by the sensori- 

 motor system. 



It is the existence of the sensori-motor system that 

 makes the animal an animal. What, then, is the 

 sensori-motor system ? It is the skeleton and muscles, 

 that is, the organs of locomotion, aggression, prehension, 

 and mastication ; the peripheral sensory and motor 

 nerves ; and the central nervous system or brain. 

 The skeleton of an animal, whether it be the carapace 

 or exoskeleton of a crustacean, or the vertebral column, 



