The luminous organs which occur in a few insects, belonging to widely
different families, and which are situated in different parts of the body,
offer, under our present state of ignorance, a difficulty almost exactly
parallel with that of the electric organs. Other similar cases could be
given; for instance in plants, the very curious contrivance of a mass of
pollen-grains, borne on a foot-stalk with an adhesive gland, is apparently
the same in Orchis and Asclepias, genera almost as remote as is possible
among flowering plants; but here again the parts are not homologous. In
all cases of beings, far removed from each other in the scale of
organisation, which are furnished with similar and peculiar organs, it will
be found that although the general appearance and function of the organs
may be the same, yet fundamental differences between them can always be
detected. For instance, the eyes of Cephalopods or cuttle-fish and of
vertebrate animals appear wonderfully alike; and in such widely sundered
groups no part of this resemblance can be due to inheritance from a common
progenitor. 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. The crystalline lens in the higher cuttle-fish consists of two
parts, placed one behind the other like two lenses, both having a very
different structure and disposition to what occurs in the vertebrata. The
retina is wholly different, with an actual inversion of the elemental
parts, and with a large nervous ganglion included within the membranes of
the eye. The relations of the muscles are as different as it is possible
to conceive, and so in other points. Hence it is not a little difficult to