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. 



V. HELIOTBOPIC 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 



