t 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 decide how far even the same terms ought to be
employed in describing the eyes of the Cephalopoda and Vertebrata. It
is, of course, open to any one to deny that the eye in either case could
have been developed through the natural selection of successive slight
variations; but if this be admitted in the one case it is clearly
possible in the other; and fundamenta