1891.] The Heliotropism of Hydra. 433 
a longtime. The results show that even after the animals have 
thoroughly established themselves in the usual position on the 
illuminated side they are to some extent continually on the march, 
and seldom remain in one spot more than a day or two, and the 
time is usually much less than this.. I cannot make out that the 
movements are more active under the red, yellow, or green, or in 
darkness, than in daylight or under blue, though a sudden change, 
whether of color or of intensity, is apt to stimulate the movements 
fora time. This latter fact probably explains the comparatively 
rapid dispersal. of the animals upon the substitution of a neutral 
color for blue (see tables), which at first sight s seems to point to a 
direct repellent action. 
On the whole, the facts seem to warrant the conclusion that 
Hydra has an innate (automatic ?) tendency to wander, and that 
light and oxygen operate not so much by calling forth new move- 
ments as by the modification of indefinite movements that tend 
continually t® recur irrespective of external stimuli. If this be so, 
the case shows an interesting analogy to the movements of plants, 
many of which (including heliotropism), as Darwin has so strikingly 
shown, have arisen through the modification by special stimuli 
of an innate circumnutatory movement. Some of these move- 
ments in plants, though no doubt unconscious, have an extraor- 
dinary likeness to purposive, intelligent acts. It would be difficult 
to say in what lies the superior claim of Hydra to i es as 
a conscious, not to say intelligent, being. 
Bryn Mawr, Pa., April, I89I. 
