THE SLUG 209 



means of all these senses then, and of certain others, we not 

 only become aware of the world about us and of what changes 

 are taking place in it, but our behavior in this world is 

 determined. 



What is true of ourselves in respect to the determination of 

 our behavior bj^ environment is true of all other animals, for 

 man is, indeed, in respect to his senses, merely one of the 

 animals. When we turn over a stone (Fig. 94), and see a 

 number of insects under it, hastening awajr until finally all 

 have become hidden again, we may be sure that this rapid 

 movement is in response to the stimulus of light, which sud- 

 denly falls upon them, and to the withdrawal of the comfortalile 

 contact with the stone above. The movement to which the 

 animals have been stimulated will be found to be, for the most 

 part, away from the light. This tends to Ijring them into a 

 darker region, and if in the neighborhood they find a crevice 

 or hole, they will come to rest there. So, too, if we look at a 

 stream full of trout, we shall find that they all stand with their 

 heads directed up-stream. They are responding to the moving 

 current which would tend to carry them away. 



When we see a ladybird beetle in the autumn climbing up 

 the side of a house, or a potato beetle climbing up a potato 

 plant, we may be sure that they are reacting to the guiding 

 stimulus of gravit.y ; and when we see flies coming toward a bit 

 of carrion, flj'ing against the wind, we may be sure that they 

 are directed in their movements hy the chemical particles 

 convejred by the wind from the carrion to their sense-organs. 



No group of animals better illustrates the principles of 

 animal behavior than the slowly moving snails and slugs. 

 They are not onlj^ sensitive to a great varietj^ of stimuli, but 

 the great deliberateness of their movements enables us to 



