PHOTOTAXIS 57 
eyes are capable of forming images. If animals are stimu- 
lated by two sources of light of the same degree. of intensity 
but of different area, the forms without eyes or in which the 
eyes produce no image turn practically as often to the one 
source of light as the other while animals with eyes producing 
a distinct image will, if positively phototactic, generally 
turn toward the light of larger area. With the perfection 
of the organs of vision the primitive phototactic tenden- 
cies of animals may become modified so as to afford the 
basis for the reactions to special objects of the visual field 
which we find in the more highly developed instincts. 
In many animals there is a strong reflex tendency to keep, 
so to speak, in statu quo with the visual field. This tendency 
accounts for many cases of so-called rheotropism as is shown 
by the experiments of Hadley on the lobster and Lyon on 
fishes. The same tendency is, as we have seen, manifested 
also in compensatory motions. Rédl has shown that in 
Daphnia and some other forms in which the eyes are mov- 
able there is an effort on the part of the eyes to become 
oriented to the rays of light. In the vertebrates in which 
the eyes are freely movable the same tendency is more or 
less pronounced. This trait may be connected with the 
involuntary tendency of animals to follow the movements 
of objects which cross their field of vision. Vision in the 
lower animals is concerned much more with the movements 
than with the form of objects. It may be possible to trace 
more accurately than has been done the relation between 
phototaxis, compensatory motions of the head and body, 
and the involuntary tendency to follow moving objects 
with the eyes. This field of investigation has been but 
little explored, but it is one which promises to be fruitful 
of significant results. 
