ey existed. Thus it is
probable that some four-gilled form was the progenitor of the dibranch
cephalopods. Now the four-gilled kinds (judging from the only existing
form, the nautilus) had the auditory organ in a very inferior condition of
development to what we find in the dibranch; thus we have not only evidence
of the independent high development of the organ in the former, but also
evidence pointing towards a certain degree of comparative rapidity in its
development.

Such being the case with regard to the organ of hearing, we have another
yet stronger argument with regard to the organ of sight, as has been well
pointed out by Mr. J. J. Murphy.[60] He calls attention to the fact that
the eye must have been perfected in at least "three distinct lines of
descent," alluding not only to the molluscous division of the animal
kingdom, and the division provided with a spinal column, but also to a
third primary division, namely, that which includes all insects, spiders,
crabs, &c., which are spoken of as Annulosa, and the type of whose
structure is as distinct from that of the molluscous type on the one hand,
as it is from that of the type with a spinal column (_i.e._ the vertebrate
type) on the other.

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In the cuttle-fishes we find an eye even more completely constructed on the
vertebrate type than is the ear. Sclerotic, retina, choroid, vitreous
humour, lens, aqueous humour, all are present. The correspondence is
wonderfully complete, and there can hardly be any hesitation in saying that
for such an exact, prolonged, and correlated series of similar structures
to have been brought about in two independent instances by merely
indefinite and minute accidental variations, is an improbability which
amounts practically to impossibility. Moreover, we have here again the same
imperfection of the four-gilled cephalopod, as compared with the
two-gilled, and therefore (if the latter proceeded from the former) a
similar indication of a certain comparative rapidity of development.
Finally, and this is perhaps one of the most curious circumstances, the
process of formation appears to have been, at least in some respects, the
same in the eyes of these molluscous animals as in the eyes of vertebrates.
For in these latter the cornea is at first perforated, while different
degrees of perforation of the same part are presented by different adult
cuttle-fishes--larg