286 STUDIES IN GENERAL PHYSIOLOGY 
also swim less easily. It is, therefore, very possible that a 
more favorable condition for the liberation of energy accom- 
panies positive heliotropism, while a more difficult condition 
accompanies negative heliotropism. 
Finally, I shall mention a circumstance which possibly 
belongs to the same group of phenomena, and which first 
led me to investigate the effects of light on animals. During 
hard glacier trips I noticed that the fatigue which set in 
disappeared at once when I removed my snow spectacles and 
exposed my eyes to the full light. On the other hand, it is 
well known that the intense light of the snow-fields increases 
the fatigue when one is exposed to it for a long time. Light 
certainly has something to do with the liberation of energy, 
either facilitating it or rendering it more difficult; and it 
seems that in certain organisms it may call forth both kinds 
of effects under different circumstances. Whether these 
observations have any deeper significance or not is to be 
determined by further experiments. 
Vv. HELIOTROPIC AND PHOTOKINETIC ANIMALS 
1. Huxley states in one of his essays that plants must 
have a nervous system, because Darwin observed reactions 
in Drosera which in some points are similar to those in ani- 
mals, In view of the identity of heliotropism in animals and 
plants, the same reasoning would force us to assume that 
plants possess eyes. The only conclusion, however, which 
may safely be drawn from these facts is that the eyes owe 
their significance for sight, among other things, to a condi- 
tion which is found also in the skin of many animals and in 
plants; namely, elements which undergo certain, but at 
present unknown, changes through light. It is not even 
necessary that these elements be everywhere entirely identi- 
cal, either physically, chemically, or morphologically. As 
is well known, certain elements are present in our retina 
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