THE TROPISMS IN GENERAL 19 
Loeb through his work “ Der Heliotropismus der Tiere” and 
various later papers. The ‘“Heliotropismus der Tiere” 
was a pioneer work; but perhaps its chief value lies not so 
much in the number of interesting facts and records of 
ingenious experiments it contains as in the general theory 
there advanced, which has been the means of stimulating 
many investigators to enter the field. 
The book emphasized the idea that it is the business of 
the investigator not merely to describe the facts of behavior 
and marvel over their wonderful nature, but to try to ex- 
plain them. It presented him with a theory which if true 
would carry us a step forward in the analysis of many 
kinds of behavior, especially in the lower organisms. The 
book shows the influence of Sachs, whose conception of the 
mechanism of tropisms in plants forms the nucleus of Loeb’s 
theory. Loeb attempted to show that the phenomena of 
Heliotropism are essentially similar in plants and animals. 
The orientation of the body to the direction of the rays, the 
fact that the more refrangible rays are the more effective in 
producing the heliotropic response, and the dependence of 
light reactions upon light intensity and temperature are 
shown by plants and animals in much the same way. All 
of these effects are supposed to rest upon certain funda- 
mental properties of living substance common to plants and 
animals. This conclusion was naturally a somewhat start- 
ling one, especially when the light reactions of highly de- 
veloped animals which were commonly supposed to be due to 
psychic causes were placed in the same category with the 
turning of leaves and stems toward the sunlight. 
The prospect of finding a mechanical explanation of the 
behavior of organisms is always an alluring one. In 
this case it proved to be especially so because the general- 
ization included such apparently diverse phenomena, and 
