THE EVOLUTION OF THE EYE. 83 



invertebrate animal buried in the skull of a vertebrate animal. 

 As it lies in its capsule, looking upwards, the lens is first seen ; it 

 forms the front boundary of a vesicle, the walls of which, starting 

 from within outwards, are made up of a layer of rods, embedded 

 in dark brown pigment, which is specially developed in front ; and 

 a double or triple row of nuclei, succeeded by a clear layer, and 

 followed by an outer layer of nuclei, composed of two or three 

 rows. These are practically the elements of the invertebrate eye, 

 and in their normal order. The relation of these parts in the eye 

 of the vertebrate animal is the exact opposite of this ; the rods 

 and cones being furthest from the cornea and from the light. 

 This is explicable by their different mode of origin embryologically, 

 but it establishes a complete difference between them. In this 

 lizard we have at the top of the pineal body a distinct molluscoid 

 or invertebrate eye." 



Now, as the pineal body is found in all the higher vertebrates 

 including man, we have a convincing proof of the descent of the 

 higher vertebrates from a form as low as that of the existing 

 Ascidian larva, as foretold by Darwin. The pineal body in all its 

 stages shows an interesting case of loss of function from disuse. 

 More highly organised eyes have been formed in the higher 

 animals, and the median eye has therefore become useless. We 

 need not go beyond our own county of Middlesex to find 

 animals with eyes becoming atrophied through disuse. It was long 

 a question with naturalists whether the common mole is or is not 

 blind. I have not got Frank Buckland's book by me, but I think 

 it was he who says, with an airy contempt for strict scientific 

 investigation, that the mole is not blind, because if you part its 

 fur you can see its eyes. Let me quote Semper again, and find 

 out the real state of the case. 



He (Semper) says : — " This animal, whose peculiar habits are 

 known to everyone, has true eyes, from which none of the 

 essential parts of the eyes of Vertebrata are absent, although these 

 parts are all of the simplest, almost of embryonic structure. The 

 whole eye is very small, deeply imbedded in muscles, and quite 

 covered by the skin, so that it is quite invisible, externally. The 

 lens consists of a very small number of minute and little altered 

 embryonic cells; the retina, in the same way, is much simpler 



