22 Bulletin Wisconsin Natural History Society. [Vol. 10, Nos. 1 & 2. 
a breeze. Perhaps the instinct of hovering shown by many kinds 
of flies is an expression of the same fundamental tendency. 
The automatic tendency to keep the body in a certain orienta¬ 
tion to its field of vision which we find among crustaceans, insects 
and lower vertebrates, 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 compensatory 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 
light house. Such orientation may be conscious and voluntary, 
but it cannot be denied that there is a sort of instinctive tendency 
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 profoundly modi' 
fied 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 
larger area which is the most potent in causing the turning of the 
body. With the development of image forming eyes it becomes 
possible for animals to respond to objects and not to mere differ¬ 
ences in the amount of light and shade. The image forming 
type of eye is stimulated by a decrease as well as by an increase 
of illumination on particular parts of its surface. This stimulation 
is generally associated with an involuntary turning toward the 
source of stimulation. Hence, the automatic turning of the head 
toward a new object in the field of vision, and the tendency to 
follow the movements of bodies with movements of the eyes. The 
eyes of animals are notoriously quick to respond to movements. 
The moving thing is the stimulating thing. With the evolution of 
the image forming type of eye and the development of acute sen¬ 
sitiveness to change of illumination of particular parts of the 
retinal surface, the general tendency to go toward or away from 
light may pass into an involuntary tendency to become oriented 
toward particular moving objects in the field of vision. When an 
