110 ; W. H. TALIAFERRO 
same in both eyes, the symmetrical muscles of the body will work 
equally, and the animal will continue to move in this direction. In 
the case of the negatively heliotropic animal the picture is the same 
except that if only one eye is illuminated the muscles connected with 
this eye will work less energetically 
Loeb holds, moreover, that the orienting stimulus in organisms, 
both animals and plants, is dependent upon the actual amount 
of stimulating energy received by the photoreceptors in accord 
with the Bunsen-Roscoe law. 
Opposed to the continuous-action theory, is the change-of- 
intensity theory, supported chiefly by the works of Jennings 
and Mast. According to this theory, the orienting stimulus 
is not dependent upon the actual amount of energy received 
by the photoreceptors, but to time-rate-of-change of the stimu- 
lating energy. Once an organism is oriented to light, it is sup- 
posed to receive no orienting stimulus until it leaves the path 
or axis of orientation. 
The great body of evidence, especially in the unicellular forms, 
tends to favor the change-of-intensity theory. Mast has dis- 
cussed this evidence fully in Light and the Behavior of Organ- 
isms (711) and in numerous recent papers (Mast, 716). In certain 
seedlings, however, Blaauw (’08), Fréschel (’08), Arisz (711), 
and Clark (13) have demonstrated that within certain limits 
orientation to light is dependent upon the actual amount of 
energy received. This is in accord with the continuous-action 
theory. Likewise, Mast (’11, p. 163) and Loeb and Ewald 
(14) have come to practically the same conclusion in regard to 
the orientation of the sessile polyp Eudendrium. 
The chief questions that are at issue between these two theories 
are: 1) Does stimulation during orientation depend upon the 
continuous action of light or to time-rate of change in the inten- 
sity? 2) Does the same stimulus that causes orientation continue 
to act after orientation? 3) Is it essential that the photoreceptors 
which receive the orienting stimulus be placed symmetrically? 
While this work was not taken up with any especial reference 
to these questions, some of the observations bear directly upon 
them. In regard to the first question, while some of the evidence 
favors the change-of-intensity theory, there is no direct evidence 
