2g4 Th e Senses of the Lower Animals . [July, 
Again, we see all objects by means of the so-called lumi- 
nous rays which they reflect. Whether they at any time 
reflect; any of the “ dark rays” is to us immaterial, because 
of such rays our vision is unable to take cognisance. But 
an animal whose retina were endowed with a wider range 
of sensibility would perceive the appearance of bodies 
modified accordingly as they reflected or absorbed such 
“ dark rays,” and would hence detedl differences between 
substances which to our sight appear absolutely alike. 
There is another point of some moment which here forces 
itself upon our attention. It is well known that animal 
organs deprived for successive generations of the oppor- 
tunity of exercising their function become abortive. Of 
this truth several of the blind animal species enumerated 
above have long served as striking instances. Why should 
the Proteus possess an eye — a light-organ — living as he 
does where no ray of light ever penetrates ? But as it has 
been pointed out by Sir Wyville Thomson in his account of 
the results of the Challenger expedition, whilst in certain 
animal groups permanent and total darkness leads to the 
total uselessness of the eyes ; in others, under precisely 
similar circumstances, the very contrary result ensues, and 
the visual organs, as in the Monida, an animal living at the 
depth of 700 fathoms below the surface of the ocean, where 
no gleam of sunlight can reach, are “ unusually developed, 
and apparently of great delicacy.” We shall have difficulty 
in conceiving in a series of successive generations the con- 
tinued development of an organ which is of no use in the 
economy of the animal. The greater weight, therefore, 
attaches to Sir W. Thomson’s suggestion that the eyes of 
deep-sea creatures under such conditions may “ become 
susceptible of the fainter light of phosphorescence.” That 
many of the inhabitants of the ocean, both when living 
and when passing into decomposition, are phosphorescent 
to an extent visible even to eyes adapted to the stronger 
stimulus of direft sunlight is indisputable. The light which 
at certain times and places follows in the wake of a ship 
and marks every stroke of a boat’s oars has often been 
described, and is due both to living animals, Salpse, 
Medusse, Pyrosomae, Nereids, &c., and to decomposing 
animal matter. So powerful is this light at times that by 
its aid fishes have been discerned at the depth of several 
feet below the surface. There can hence be no doubt that 
an eye sensitive even to the faintest phosphorescence would 
be of great service to marine animals, enabling them either 
to discover their prey or to shun an approaching enemy. 
