366 THE MAINTENANCE OF THE INDIVIDUAL 



appear and walking takes place. The second child has both condi- 

 tions present. 



This explanation of the display of energy helps us to understand the 

 mental life of higher animals, especially with reference to a directed 

 urge toward definite goals of behavior. In the pages that follow an 

 attempt will be made to show how conscious life has developed. No 

 set theories or beliefs will be imposed on the reader, but a brief presen- 

 tation of the facts will be given as we see them. The student can 

 then do his own thinking. 



Why Living Things Are Responsive 



Life has been likened by many writers to a flowing river which 

 continually moves in one direction. Meeting obstacles, it is diverted 

 from its course, moving rapidly over steep declivities and meandering 

 slowly in level valleys. We do not think of a river in terms of water 

 alone, but also in terms of the rocks in its bed, of its banks of gravel 

 or soil, even of the forests in which it takes its source, and of the 

 wharves and bridge abutments of the cities through which it passes 

 in its course. We know that eddies in the river mark submerged 

 rocks, that sharp curves may be caused by areas too hard for the river 

 to erode, that ledges may cause waterfalls. It is not possible to think 

 of the river without the environment which surrounds it. 



Guided by this comparison, we note the cause of sensitiveness of 

 living matter of which an organism is made up in the fact that wher- 

 ever factors of the environment impinge upon the organism, changes 

 in the latter are sure to take place. These factors, forces, or things 

 that cause changes in the life activities of plants or animals are 

 called stimuli, and changes in relation between the organism and its 

 surroundings, reactions to stimuli. Such responses may be sudden, 

 as the involuntary start which comes as a result of some unexpected 

 noise or the quick withdrawal of one's hand from a hot object, or 

 they may be extremely slow and continuous, as is seen in the gradual 

 turning movements of a plant placed in an area of unequal illumina- 

 tion. The sum total of all the reactions of an organism to the stimuli 

 which impinge upon it constitutes its behavior. 



Various Kinds of Stimuli 



In order to understand what causes behavior, we must analyze the 

 various kinds of stimuli which act upon plants and animals, as follows : 



