104 MENTAL EVOLUTION IN ANIMALS. 



is completed. In the case of the eye, again, the earliest sign 

 of commencement consists in a similar pitting of the general 

 integument, but the lining of this pit is not destined, as in 

 the previous cases, to become the receptive surface of the 

 sensory impressions. For, after it has deepened considerably 

 it undergoes sundry changes which result in its forming the 

 cornea, aqueous humour, and crystalline lens; while the 

 retina arises as an offshoot from the brain in the form of a 

 sac growing, as it were, upon a slender stalk towards the 

 crystalline lens. At first the anterior surface of this sac is 

 convex, but the posterior part afterwards becomes pushed 

 into the cavity of the sac ; so that the anterior surface 

 eventually becomes strongly concave. Therefore the sac is 

 now, as Professor Huxley graphically describes it, "like a 

 double night-cap, ready for the head, but the place which the 

 head would occupy is taken by the vitreous humour, while 

 the layer of night-cap next it becomes the retina." Thus the 

 rods and cones of the retina are not developed immediately 

 out of the epidermic cells of the integument ; but inasmuch as 

 the brain is itself begun as an infolding of the epidermic layer, 

 the rods and cones of the retina are ultimately derived from 

 those epidermic cells. Or, again to quote Professor Huxley, 

 "the rods and cones of the vertebrate eye are modified 

 epidermal cells, as much as the crystalline cones of the insect 

 or crustacean eye are."* Therefore, in the words of Professor 

 Hreckel, " the general conclusion has been reached that in 

 man, and in all other animals, the sense-organs as a whole 

 arise in essentially the same way, viz., as parts of the external 

 integument or epidermis. The external integument is the 

 original general sense-organ. Gradually the higher sense- 

 organs detach themselves from this their primal condition, 

 whilst they withdraw more or less completely into the pro- 

 tecting parts of the body. Nevertheless in many [inverte- 

 brate] animals, even at the present hour, they lie in the 

 integument, as e.g., in the Vermes." 



I have entered thus fully into this general fact, because 

 it is of importance, not only to the theory of evolution, but 

 also to the philosophy of sensation, to know from such direct 

 historical sources that all the special senses are differentiations 

 of the general sense of touch. 



* Science and Cullure, &c, p. 271. 



