16 Bulletin Wisconsin Natural History Society. [Vol. 3 0, Nos. 1 & 2. 
mals it is the rays nearer the violet spectrum that are the more 
potent in evoking the heliotropic response. In both plants and 
animals temperature, previous exposure to light and other external 
factors influence reactions to light in much the same way. Back 
of all the differences of form and function of plants and animals, 
and notwithstanding the higher organization of the animal world 
with its specialized sense organs and complex nervous systems, 
the living substance of organisms possesses certain fundamental 
common properties of irritability upon which the common and fun¬ 
damental features of behavior which we call tropisms depend. 
The theory of Loeb would sweep away all higher psychic fac¬ 
tors in the realm of tropisms, and reduce the phenomena to com¬ 
paratively simple manifestations of reflex irritability. Further he 
would explain much of the so-called instincts of animals as a result 
of these tropisms. Since the prospect of finding a mechahical or 
causal explanation of any feature of behavior is always an allur¬ 
ing one, it will be of interest to pass in review some of these cases 
of tropisms with the end of determining how far the reflex theory 
will carry us. And then we shall consider the relation of these 
tropisms to more complex forms of behavior. 
An excellent illustration of tropism is afforded by the light 
reactions of the larvae of the marine worm Arenicola. These larvae 
are oblong in shape with two eye spots at the anterior end. Near 
either extremity there is a band of cilia by means of which the 
larvae swim through the water. The larvae are positive in their 
reactions to light, and will follow a light around in various direc¬ 
tions. Orientation to light is brought about by bending the body 
to the stimulated side. If the larvae is between two sources of 
light from which the rays intercept one another as they fall on 
the animal at an angle of ninety degrees, the larva will take a course 
midway between the two lights. If one light is turned off the 
larva bends immediately to the other one. By arranging a mirror 
so as to throw a small spot of light on different parts of the body 
Mast has shown that when light is thrown into one eye there is a 
strong bend of the body toward the stimulated side. The parts 
behind the eye spot show no definite reaction. It is evident that 
orientation in this form is due to different intensities of illumina¬ 
tion on two sides of the body. So far as can be ascertained orien¬ 
tation takes place directly and automatically, without any con- 
