14 Bulletin Wisconsin Natural History Society. [Vol. 10, Nos. 1 & 2. 
ponent factors. The analytical study of behavior is simply a con¬ 
sequence of .extending to animal psychology the methods of 
experimental investigation so largely employed in the physical 
sciences and which are coming more and more to be employed in 
biology and in the laboratory investigations of the psychology of 
man. The results thus far won may be meagre, but judging from 
the increasing number of trained investigators who are devoting 
themselves to the work, we may look forward to a rapid increase 
in our knowledge and insight. 
From the standpoint of analysis the subject of tropisms is one 
of great import. Certain stimuli exercise a direct effect upon 
the movements of animals, causing them to go toward or away 
from the source of stimulation. The moth flies toward a candle ; 
infusorians gather in regions of dilute acids and avoid regions of 
too great heat or cold ; certain caterpillars tend to crawl opposite 
the direction of the force of gravity. These directed movements 
are commonly called tropisms but there is a variety of opinions 
regarding the kinds of behavior to which the term tropism may 
be applied and usage has not settled authoritatively upon any rigid 
definition of the word. We shall therefore use the word in a 
somewhat broad and indefinite sense. 
Tropisms have long been recognized in plants. The familiar 
phenomenon of the turning of plants to the sun was termed helio- 
tropism by De Candolle in 1835, and he, in common with several 
other botanists in the early and middle parts of the nineteenth 
century, regarded this turning as a direct and more or less mechan¬ 
ical effect of sunlight upon the tissues of the plant. Sachs on 
the other hand emphasized the aspect of irritability in tropisms. 
and maintained that it is the direction in which the rays of light 
penetrate the tissues of the plant and not merely the different 
degrees of illumination on the two sides, that determines the direc¬ 
tion of turning. The work of Sachs on plants directed the atten¬ 
tion of Loeb to the phenomena of tropisms in animals, and also 
furnished him with some of the fundamental conceptions of his 
own celebrated theory. Loeb’s first paper on tropisms of any con¬ 
siderable length was entitled The Heliotropism of Animals and its 
Agreement with the Heliotropism of Plants. The publication of 
this paper marked an epoch in the analytical study of animal 
behavior. Previous to this time the tropic responses of animals 
were interpreted as the expression of the predilections or con- 
