142 DIFFICULTIES OF THE THEORY [CHAP. VI 



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 cephalo- 

 pods or cuttle-fish and of vertebrate animals appear wonderfully 

 alike; and in such widely sundered groups no part of this re- 

 semblance 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 Cepha- 

 lopoda 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 inde- 

 pendently 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 a common progenitor. 



Fritz Miiller, 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. In two of these families, which were more especially 

 examined by Miiller, 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 within their complex stomachs, and lastly in the whole 



