SENSATION. 85 



not correct. " We do not really see things reversed, for the 

 mind is not a perpendicular object in space, standing behind 

 the retina in the manner that a photographer stands behind 

 his camera. To the mind there is no up or down in the 

 retina, except in so far as the retina is in relation to the 

 external world ; and this relation can only be determined, 

 not by sight, but by touch. And if only this relation is 

 constant, it can make no difference to the mind whether the 

 images are direct, reversed, or thrown upon the retina at any 

 angle with reference to the horizon ; in any case the corre- 

 lation between sight and touch would be equally easy to 

 establish, and we should always sec things, not in the position 

 in wdiich they ai'e thrown upon the retina, but in that which 

 they occupy with reference to the retina. Thus it really re- 

 quires no more ' practice' correctly to interpret inverted 

 images than it does similarly to interpret upright images ; 

 and therefore the fact that some eyes of an ant are sup- 

 posed to throw direct images, wdiile others are supposed to 

 throw inverted, is not any real objection to the theory" 

 that they do.* 



There is no one group in the animal kingdom where we 

 have so complete a series of gradations in the evolution of an 

 organ of special sense as is presented by the organ of sight in 

 Worms. " In the lowest Vermes," — I quote from Professor 

 HcTeckelf — "the eye is only made up of individual pigment-cells. 

 In others, refractive bodies are associated with these, and form 

 a very simple lens. Behind these refractive bodies sensory 

 cells are developed, forming a retina of the simplest order 

 presenting a single layer, the cells of which are in connection 

 with extremely delicate terminal fibres of the optic nerve. 

 Lastly, in the Alcipidse, which are highly organized Annelidas 

 that swim on the surface of the sea, adaptation to this mode 

 of life has brought about such perfection of the eye that this 

 organ in these animals is in no way inferior to that of the 

 lower vertebrata. In these creatures we find a large globular 

 eye-ball, enclosing externally a laminated globular lens, 

 internally a vitreous body of large circumference. Imme- 

 diately investing these are rods of the usual cells sensitive to 

 light, which are separated by a layer of pigment-cells from 

 the outer expansion of the optic nerve or retina. The ex- 



* Quoted from an article of my own in Nature, June 8, 1882. 

 t Essay on Origin and Development of Sense-organs. 



