THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 63 
Eurypteride—in fact, in the Paleostraca generally—there are two 
median eyes in addition to the lateral eyes, which were innervated 
from these ganglia. 
In Ammoceetes, then, if the supra-infundibular portion of the 
brain really corresponds to the supra-cesophageal of the paleostracan 
group, we ought to find, as indeed is the case, an optic apparatus 
consisting of two lateral eyes and two median eyes, innervated from 
the supra-infundibular brain-mass, and an olfactory apparatus built 
up on the same lines as in the scorpion-group, also innervated from 
this region. If, in addition, it be found that those two median eyes 
are degenerate eyes of the same type as the median eyes of Limulus 
and the scorpion-group, then the evidence is so strong as to amount 
to a proof of the correctness of the theory. This evidence is precisely 
what has been obtained in recent years, for the vertebrate did possess 
two median eyes in addition to the two lateral ones, and these two 
median eyes are degenerate eyes of the type found in the median 
eyes of arthropods and are not of the vertebrate type. Moreover, as 
ought also to be the case, they are most evident, and one of the 
pair is most nearly functional in the lowest perfect vertebrate, 
Ammoceetes. 
Of all the discoveries made in recent years, the discovery that 
the pineal’ gland of the vertebrate brain was originally a pair of 
median eyes is by far the most important clue to the ancestry of 
the vertebrate, for not only do they correspond exactly in position 
with the median eyes of the invertebrates, but, being already 
degenerate and functionless in the lowest vertebrate, they must have 
been functional in a pre-vertebrate stage, thus giving the most direct 
clue possible to the nature of the pre-vertebrate stage. It is 
especially significant that in Limulus they are already partially 
degenerated. What, then,-ought to be the structure and relation to 
the brain of the median and lateral eyes of the vertebrate if they 
originated from the corresponding organs of some one or other member 
of the paleostracan group ? 
This question will form the subject of the next chapter. 
SUMMARY. 
The object of this book is to attempt to find out from what group of inverte- 
brates the vertebrate arose ; no attempt is made to speculate upon the causes of 
variation by means of which evolution takes place. 
