Tropisms — Relation to Modes of Behavior 65 



stream. Many insects show the same trait in their 

 flying against a breeze. Perhaps the instinct of 

 hovering shown by many kinds of flies is an expres- 

 sion of the same fundamental tendency. 



The automatic tendency to keep the body in a 

 certain orientation to its field of vision which we 

 find among crustaceans, insects and lower verte- 

 brates, is to a greater or less extent replaced in 

 forms with freely movable eyes by ocular movements 

 which enable the moving animal to retain the same 

 field of vision. Stalk-eyed crustaceans show com- 

 pensatory movements of the eye stalks. Similar 

 eye movements occur in fishes, amphibians and birds. 

 A man at night more or less involuntarily directs 

 his steps toward a single light in his horizon much 

 as birds are drawn toward a lighthouse. Such ori- 

 entation may be conscious and voluntary, but it can- 

 not be denied that there is a sort of instinctive ten- 

 dency toward it much as there is in all of us a strong 

 tendency toward a certain orientation to the force 

 of gravity. 



The reactions of animals to light have been pro- 

 foundly modified by the evolution of the image- 

 forming eye. It has been shown by Cole that if 

 an eyeless form such as an earthworm or a form 

 with simple eyes is subjected to stimulation by two 

 sources of light of equal intensity but of different 

 area, the animal is as likely to turn to the smaller 

 light as to the larger one. In forms with image- 

 forming eyes, on the other hand, it is the light of 



