In Ammocoetes, then, if the supra-infundibular portion of the brain really corresponds to the supra-oesophageal of the palæostracan 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, Ammocoetes. 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 palæostracan 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 invertebrates the vertebrate arose; no attempt is made to speculate upon the causes of variation by means of which evolution takes place.