ORIGIN OF SPECIES 99 



body, we must understand that there has been a great shift- 

 ing of functions among the organs. The stomach, the liver, 

 and, in a certain sense, the lungs are new. Indeed our 

 thyroid body the two lobes at the side of the larnyx, 

 which sometimes hypertrophy into goitre, an organ of 

 which hitherto neither anatomists nor physiologists have been 

 able to give an explanation is a disused reproductive organ 

 of our invertebrate ancestors. Our two eyes may be repre- 

 sented in the ocelli and lateral eyes of some invertebrates, 

 but they are not their median eyes. The eye of an octopus 

 looks very much like that of a fish, but it has long been 

 known that it is constructed on quite a different plan. In 

 the eye of the octopus the rods and cones, the elongated 

 cells which are sensitive to light, are directed forwards ; 

 in the fish they are directed backwards, and the front of 

 the retina consists of a sheet of transparent nerve fibres. 

 The eye of the vertebrate is, therefore, the invertebrate 

 eye turned inside out. Ingenious hypotheses have been 

 formulated to explain how this puzzling involution came 

 about. There is no need of any hypothesis, according to 

 Dr. Gaskell. Deeply seated in the centre of our brain is 

 a little conical organ, the pineal body, which has acquired 

 a spurious fame, because Descartes, looking at it in its 

 relation to the great hemispheres of the brain and the 

 cerebellum which overarch it, and thinking how closely it 

 resembled an organist seated at an organ, imagined that it 

 might be the seat of the soul. The pineal body in certain 

 curiously archaic reptiles, particularly Hatteria punctata of 

 New Zealand, has a long stalk and reaches to a hole in the 

 roof of the skull, which is closed by semi-transparent mem- 

 brane, and in structure it is most clearly an eye formed on 

 the invertebrate plan. Hatteria has two inverted eyes, as we 

 may call them, as well as its cyclopean pineal eye in the 

 middle of its head ; but Dr. Gaskell has shown from its 

 development in the larva of the lamprey, that the pineal eye 

 was formerly paired, and that its connections with the brain 



