These complex arrangements of parts could not have been evolved by "Natural Selection," _i.e._ by minute accidental variations, except by the action of such through a vast period of time; nevertheless, it was fully evolved at the time of the deposition of the upper Silurian rocks.
Cuttle-fishes (_Cephalopoda_) are animals belonging to the molluscous primary division of the animal kingdom, which division contains animals formed upon a type of structure utterly remote from that on which the animals of the higher division provided with a spinal column are constructed.

And indeed no transitional form (tending even to bridge over the chasm between these two groups) has ever yet been discovered, either living or in a fossilized condition.[58] Nevertheless, in the two-gilled Cephalopods (_Dibranchiata_) we find the brain supported and protected by a cartilaginous cranium.

In the base of this cranium are two cartilaginous chambers.

In each chamber is a membranous sac containing an otolith, and the auditory nerves pass from the cerebral ganglia into the cartilaginous chambers to reach the auditory sacs.

Moreover, it has been suggested by Professor Owen that {75} sinuosities between processes projecting from the inner wall of each chamber "seem to be the first rudiments of those which, in the higher classes (_i.e._ in animals with a spinal column), are extended in the form of canals and spiral chambers, within the substance of the dense nidus of the labyrinth."[59] [Illustration: CUTTLE-FISH.
A.Ventral aspect.