Seite - 200 - in The Origin of Species

200 ORIGIN OF SPECIES 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 in- stance 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 al- most as remote as is possible amongst flowering plants; but here again the parts are not homologous. In all cases of be- ings, far removed from each other in the scale of organisa- tion, 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 dif- ferences between them can always be detected. For instance, the eyes of cephalopods or cuttle-fish and of vertebrate ani- mals 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. Be- yond 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 dift'erent structure and disposition to what occurs in the vertebrata. The retina is wholly dif- ferent, with an actual inversion of the elemental parts, and with a large nervous ganglion included within the mem- branes of the eye. The relations of the muscles are as dif- ferent 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