176 NATURAL SCIENCE. March. 



or lived in tubes into which they could withdraw, their times of 

 exposure being when feeding. The significance of this the author, 

 no doubt correctly, points out. Warning of the approach of an enemy 

 would in most cases be first given by its shadow. Hence this 

 " Schattenempfindung " is most probably protective. On the other 

 hand, the sensitiveness to light-increase is most prominent in animals 

 which live in sand or mud, from which they occasionally and, as it 

 were, shyly emerge. Their sensitiveness to light serves to keep them 

 near their retreats. Amphioxus is a good example of this. It lives in 

 sand, and is violently agitated by light if it cannot bury itself. It was 

 originally thought that the sensitiveness to light of this animal might 

 be due to the pigment spot at the anterior end of the nerve cord, but 

 "headless " specimens are equally shy of light. This appears to limit 

 the seat of the sensation to the skin. 



These very variations of the function, in more or less evident 

 adaptation to the biological needs of the animals, are, it seems to me, 

 a further indication of its secondary origin, and we are led to believe 

 that, if need be, any skin could acquire the capacity of reacting, either 

 to sudden increases or to sudden diminutions of light-intensity if its 

 life-condition required it. I say we are led to believe this simply 

 because so many different animals have developed it, and quite apart 

 from any theory as to the physiological principles involved ; we shall 

 return to these latter later on. 



We noted above that Rawitz was rather inclined to doubt the 

 existence of a " Schattenempfindung," his reason being, that if light 

 were a stimulus, the negation of light could not be so at the same time. 

 Dr. Nagel discusses the point in one of his appendices. He rightly 

 points out that the fallacy in this argument lies in supposing that the 

 light-waves as such are felt, i.e., give rise to some corresponding 

 vibrations in the nerves. This is certainly not the case, and all we 

 can rightly assume is that certain potential energies exist within the 

 living substance, and that these are brought into play by the action of 

 light, the resulting molecular movements having no necessary likeness 

 to light-waves. From this point of view, changes in the molecular or 

 atomic movements within the living substance, so far as we can see, 

 could be produced as well by sudden illumination as by sudden 

 shadows. Such changes are all, then, that we require for the 

 production of the sensation. Further, Dr. Nagel's answer to Rawitz 

 might have been still more complete if he had pointed out that the 

 sudden diminution of light most certainly causes a distinct sensation 

 with us. A sudden shadow always startles us. In view of the 

 justifiable correlation between the specialised organ for recording 

 variations in light-intensity, and the skin possessed of the dermatoptic 

 function, there seems to be no reason why, in such essential matters, 

 we cannot assume that what is true of the one will be true of the 

 other also. Indeed, as above noted. Dr. Nagel has experimentally 

 established this shadow-sensation in oysters. 



