212 Dr. J. Berry Haycraft. 



Sp.C. = Spinal cord. 



III.LO 



IV.L. I 



V.L. \- 3rd to 7th lumbar nerve roots. 

 YI.L. I 

 YILL.J 



T Qj "\ 



II S I = ^ 8 * an( ^ ^ nc ^ sacra ^ nerve roots. 



E.G. = External cutaneous nerve. 



A.C. = Anterior crural nerve. 



Obt. = Obturator nerve. 



Sc. = Sciatic nerve. 



x = Branch from 5th to 6th lumbar nerve root. 



^*A New Hypothesis concerning Vision." By JOHN BERRY 

 HAYCRAFT, M.D., D.Sc. Communicated by E. A. SCHAFER, 

 F.R.S. Received February 16, Read March 2, 1893. 



(Abstract.) 



It is suggested that many of the well-known facts of vision can be 

 more easily understood when studied from the evolutionary stand- 

 point. The eye is no exception to the general rule, accepted by 

 evolutionists, that all parts of the body are gradually evolved under 

 the environmental conditions of the species. Many species are 

 devoid of a colour sense, but are able, nevertheless, to distinguish 

 light from darkness, and where a colour sense is present it has been 

 developed in relationship with environmental pigments : these points 

 have been brought out with especial clearness by Darwin and 

 Lubbock. We may infer, therefore, that the visual apparatus of a 

 colour-seeing species man, for instance was at one time only able 

 to distinguish light from darkness, and that the colours red, yellow, 

 green, &c., were once seen as grey. This enables us to understand 

 why it is that the outer, less used, parts of the retina are at the 

 present day colour blind ; this fact fits in at once with our evolution- 

 ary hypothesis. From the same point of view we may explain why 

 a minimal stimulus from a red, green, or other coloured object gives 

 rise merely to the sensation grey " bei Nacht sind alle Katzen 

 grau" even when it falls upon the centre of the retina. In this 

 case the minimal stimulus is unable to excite more than the simple 

 sensation of light, and the quality of this light is not seen. A 

 parallel may in fact be drawn between sight and hearing and smell, 

 for we may hear a sound too feebly to assign to it its pitch, and we 

 may have to sniff a faint odour in order to make out exactly what it 

 is. But a red, yellow, or green object, very brightly illuminated, 

 also appears white, and this has been explained in various ingenious 

 ways. It is suggested, however, that this is merely a special case of 



