74 THE ORIGIN OF VERTEBRATES 



retina, as quoted from Bobretsky by Balfour, is therefore iu reality 

 the development of the retinal ganglion, and not of the retina proper. 

 There is, I imagine, a universal belief that the natural mode of 

 origin of a sense-organ, such as the eye, must always have been from 

 the cells forming the external surface of the animal, and that direct 

 origin from the central nervous system is a priori most improbable. 

 It is, therefore, a matter of satisfaction to find that the evidence for 

 the latter origin has universally broken down, with the single 

 exception of the eyes of vertebrates and their degenerated allies ; a 

 fact which points strongly to the probability that a reconsideration 

 of the evidence upon which the present teaching of the origin of the 

 vertebrate eye is based will show that here, too, a confusion has 

 arisen between that part formed from the epidermal surface and that 

 from the optic ganglion. 



The Median or Pineal Eyes. 



Undoubtedly, in recent times, the most important clue to the 

 ancestry of vertebrates has been given by the discovery that the 

 so-called pineal gland in the vertebrate brain is all that remains of a 

 pair of median or pineal eyes, the existence of which is manifest in 

 the earliest vertebrates ; so that the vertebrate, when it first arose, 

 possessed a pair of median eyes as well as a pair of lateral eyes. 

 The ancestor of the vertebrate, therefore, must also have possessed a 

 pair of median eyes as well as a pair of lateral eyes. 



Very instructive, indeed, is the evidence with regard to these 

 median eyes, for one of the great characteristics of the ancient 

 palreostracan forms is the invariable presence of a pair of median 

 eyes as well as a pair of lateral eyes. In the living representative of 

 such forms — Limulus — the pair of median eyes (Fig. 5) is well 

 shown, and it is significant that here, according to Lankester and 

 Bourne, these eyes are already in a condition of degeneration ; so 

 also in many of the Paheostraca (Fig. 7) the lateral eyes are the large, 

 well- developed eyes, while the median eyes resemble those of Limulus 

 in their insignificance. 



We see, then, that in the dominant arthropod race at the time 

 when the fishes first appeared, the type of eyes consisted of a pair of 

 well-developed lateral eyes and a pair of insignificant, partially 

 degenerated, median eyes. Further, according to all palaeontologists, 



