e in the calamaries, smaller in the octopods, and
reduced to a minute foramen in the true cuttle-fish sepia.

Some may be disposed to object that the conditions requisite for effecting
vision are so rigid that similar results in all cases must be independently
arrived at. But to this objection it may well be replied that Nature
herself has demonstrated that there is no such necessity as to the details
of the process. For in the higher Annulosa, such as the dragon-fly, we meet
with an eye of an unquestionably very high degree of efficiency, but formed
on a type of structure only remotely comparable with that of the fish or
the cephalopod. The last-named animal might have had an eye as efficient as
that of a vertebrate, but formed on a distinct type, instead of being
another edition, as it were, of the very same structure.

In the beginning of this chapter examples have been given of the very {78}
diverse mode in which similar results have in many instances been arrived
at; on the other hand, we have in the fish and the cephalopod not only the
eye, but at one and the same time the ear also similarly evolved, yet with
complete independence.

Thus it is here contended that the similar and complex structures of both
the highest organs of sense, as developed in the vertebrates on the one
hand, and in the mollusks on the other, present us with residuary phenomena
for which "Natural Selection" alone is quite incompetent to account. And
that these same phenomena must therefore be considered as conclusive
evidence for the action of some other natural law or laws conditioning the
simultaneous and independent evolution of these harmonious and concordant
adaptations.

Provided with this evidence, it may be now profitable to enumerate other
correspondences, which are not perhaps in themselves inexplicable by
Natural Selection, but which are more readily to be explained by the action
of the unknown law or laws referred to--which action, as its necessity has
been demonstrated in one case, becomes _a priori_ probable in the others.

[Illustration: SKELETON OF AN ICHTHYOSAURUS.]

Thus the great oceanic Mammalia--the whales--show striking resemblances to
those prodigious, extinct, marine reptiles, the Ichthyosauria, and this not
only in structures readily referable to similarity of habit, but in such
matters as greatly elongated premaxillary bones, together with the
concealment of certain bones of the skull by other cranial bones. [P