Origin of Species Variorum

Comparison with 1872

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 fundamental differences of structure in the visual organs of two groups might have been anticipated, in accordance with this view of their manner of formation. As two men have sometimes independently hit on the same ... invention, so in the several foregoing cases it appears that natural selection, working for the good of each being, and taking advantage of all favourable variations, has produced similar organs, as far as function is concerned, in distinct organic beings, which owe none of their structure in common to inheritance from the same ancestor.

Fritz Müller, in ... order to test the conclusions arrived at in this volume, has followed out with much care a nearly similar line of argument. Several families of crustaceans include a few species, possessing an air-breathing apparatus and fitted to live out of the water and possess an air-breathing apparatus. In two of these families, which were more especially examined by Müller, and which are nearly related to each other, the species agree most closely in all important characters: namely in the structure of their sense-organs, in their heart and system of circulation, in the position of every tuft of hair within their complex stomachs, and lastly in the water-breathing branchiæ, even to the microscopical hooks by which they are cleansed. Hence it might have been expected that in the few species belonging to both families which live on the land, the equally-important air-breathing apparatus would have been the same; for why should this one apparatus, given for the same .. purpose, have been made to differ, whilst all the other important organs were closely similar or rather identical. .. .. ...

Fritz Müller then argued to himself that this close similarity in so many points of structure must, in

identically the same, yet some fundamental difference between them can always, or almost always, be detected. ↑ I am inclined to believe that, in the same manner as two men have sometimes independently hit on the same invention, so natural selection, working for the good of each being and taking advantage of analogous variations, has sometimes modified in nearly the same way two organs in two distinct organic beings, which owe but little of their structure in common to inheritance from the same ancestor.

Fritz Müller, in a remarkable work recently published, has investigated a nearly parallel case, in order to test the views advanced in this volume. Several families of crustaceans include a few species which possess an air-breathing apparatus, and are fitted to live out of the water. .. .. .. .. .. In two of these families, which were more especially examined by Müller, and which are nearly related to each other, the species agree most closely in all important characters; namely in ... their sense-organs, circulating system, in the position of the tufts of hair with which their complex stomachs are lined, and lastly in the whole structure of the water-breathing branchiæ, even to the microscopical hooks by which they are cleansed. Hence it might have been expected that the equally important air-breathing apparatus would have been the same in the few species in both families which live on the land; and this might have been the more confidently expected by those who believe in distinct creations; for why should this one apparatus, given for the same special purpose to these species, have been made to differ, whilst all the other important organs are closely similar or rather identical.

Fritz Müller .. argues .. that this close similarity in so many points of structure must, in