74 THE ORIGIN OF VERTEBRATES 
retina, as quoted from Bobretsky by Balfour, is therefore in 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 @ 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. 
Tue MepIAN 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 
paleostracan 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 Palwostraca (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 paleontologists, 
