432 The American Naturalist. [May, 
guided by differentiated visual organs or not. If this conclusion 
be well founded, the efficiency of the blue rays must depend upon 
some fundamental characteristic of protoplasmic action, and the 
sensibility to the lower rays, as manifested by differentiated visual i 
end-organs in higher forms, has probably been secondarily 
acquired by an extension of the original blue-sensibility. | 
It seems hardly necessary to point out that this conclusion by | 
no means implies that all forms of heliotropic action have the ; 
same physiological meaning. It relates solely to the mode of 
stimulus, not to the purpose of the actions called forth by the 
stimulus. Sneezing and winking may both be produced bya 
sudden visual stimulus, but we do not for this reason conclude 
that these actions must play the same physiological vé/e. 
To the ultra-violet rays the animals, as far as can be deter- 
mined, are as indifferent as to the ultra-red. - f 
V.—The last point to be considered relates to the mode in which | 
the stimulus acts,—a question of greater importance than appes 
at first sight. There seems to be no doubt that blue rays imping- : 
ing upon Hydra exert a directly attractive influence; for if an 
aquarium be supplied with blue light only (entering through a 
small window) the animals move pretty directly towards it, and : 
do not simply wander aimlessly about until they reach the blue : 
by accident. The case is different when a number of the ani- 
mals, already situated on the illuminated side of a square aqu 
rium, are offered the choice between a number of differs 
colored slips fastened to that side. a 
Under these conditions, as has been shown, the animals 
decrease under the red, yellow, and green glasses, and steadily 
accumulate under the blue, although no unmixed blue light = a 
pinges upon those individuals not actually behind the blue a oe 
The lower rays, however, exert no repellant action in themselves, 
and we must therefore assume that the animals tend to wander oe 
irregularly about until the blue areas are accidentally discat : 
Observation shows, moreover, that the tendency to wan : 
under every condition of illumination. By marking off the oe 
of an aquarium into small squares it is easy to follow „i 
rately record the individual movements of a group of Hydras 
