The Senses of Animals. 295 



aetherial vibrations. Special refraction-globules would also 

 act as minute lenses, focussing the light, and thus concen- 

 trating it upon certain spots. 



In many of the lower animals we find such organs, 

 belonging to our first category, and constituting either eye- 

 spots of pigmented material or simple lenses covering a 

 pigmented area. If we call these eyes, we must remember 

 that in all probability they have no power of what we call 

 vision only a power of distinguishing light from dark. 

 Where, however, there exists beneath the lens a so-called 

 retina, that is, 1 a layer of rod-like endings of a nerve, it 

 might, at first sight, be thought that there, at any rate, we 

 have true vision. But in all probability, in a great number 

 of cases the retinal rods are simply for the purpose of 

 rendering the organism sensitive, not only to the presence 

 of light, but to its direction. Light straight ahead (a) stimu- 

 lates the middle rods ; from one side (b, c) it is focussed on 

 the rods of the opposite side of 

 the retina ; and similarly for 

 intermediate positions. The 

 presence of a retinal layer is 

 thus no infallible sign of a 

 power of vision as apart from 

 mere sensibility to light. In- 

 deed, in a great number of 



, Fig. 39. Direction-retina. 



cases, from the convexity and Simple retina for distinguishing the dlrection 



position Of the lens, the for- of the source of light or of shadow. 



mation of an image is impossible. Only when it can be 

 shown that a more or less definite image can be focussed 

 on the retina, or can be formed on the principle of mosaic 

 vision, can we justly surmise that a power of true vision 

 is present. I doubt whether this can be shown to be 

 unquestionably the case in any forms but the higher 

 arthropods, the cuttle-fishes and their allies, and the verte- 

 brates. 



There is one more point for consideration before we 

 leave the sense of sight Are the limits of vision the same 

 in the lower forms of life as they are in man ? or, to put 



