Mr. Mivart has advanced this case as one of special difficulty, but I am unable to see the force of his argument.

An organ for vision must be formed of transparent tissue, and must include some sort of lens for throwing an image at the back of a darkened chamber.

Beyond this superficial resemblance, there is hardly any real similarity between the eyes of cuttle-fish and vertebrates, as may be seen by consulting Hensen's admirable memoir on these organs in the Cephalopoda.

It is impossible for me here to enter on details, but I may specify a few of the points of difference.