130 ANIMAL BEHAVIOUR 



things, say a cyclonic disturbance, reacts with the other things 

 that environ it. The tendency of the inanimate reactions is 

 towards disorganization : thus the cyclone dissipates itself. But 

 the tendency of the activities of an organism, or race of organisms, 

 is towards their maintenance, and even their increase in numbers, 

 and towards ubiquity of distribution. This tendency is what we 

 mean by the purposes of animal behaviour. 



46. ON THE LIFE-URGES 



It is convenient arbitrarily to decompose life-activity into the 

 urges or elementary biological categories. These are assimilation, 

 growth and reproduction, and individual self-preservation : they 

 are manifested in behaviour. 



46^. Assimilation. By this we mean that the organism 

 selects and absorbs materials from its environment, makes these 

 similar to, and incorporates them with, the materials of its own 

 body. At its clearest, assimilation is exemplified by the way in 

 which the green plant absorbs COo, OH 2 and inorganic nitrogen- 

 compounds from the atmosphere and soil and, by making use 

 of the degrading energy of solar radiation, synthesizes these into 

 sugar, starch, cellulose, proteins, oils, etc., all of which materials 

 are then reassembled as the tissues of the living plant. So 

 also with other plant and animal organisms in a host of different 

 ways. 



Assimilation provides the animal organism with materials that 

 can be oxidized in the processes of metabolism. It is thus that 

 the energy for bodily, behaviouristic motions is obtained. But 

 assimilation is also necessary for growth of the individual body 

 and for reproduction. When, however, an animal has ceased to 

 grow and is not reproducing it assimilates in order to obtain 

 the energy for its behaviour. 



^6h. Growth and Reproduction. The organism tends con- 

 tinually to increase in magnitude and in power over its environ- 

 ment — this is its growth in the most general sense. Simple growth 

 means just increase in bodily magnitude without appreciable 

 change in bodily form, and there are short phases in the life- 

 histories of all organisms when simple growth proceeds. 



Developmental tectonics. In its embryogeny (see Sections 70, 

 72) the organism assimilates and, in a sense, reproduces. The 



