80 COSMIC PHILOSOPHY. [pt. ii. 



But the only special resemblances which are found to exist, 

 are those between the eyes and the ears. Now these are 

 organs in which such variations as occur must be in a pre- 

 eminent degree directly adaptive. The eye, for example, 

 contains an optical apparatus of which the function is the 

 concentration of rays of light into a focus upon the retina. 

 Such is the function discharged by the lens, and the vitreous 

 and aqueous humours. Now, while the compound eyes of 

 insects show us that this function can be discharged in more 

 than one way, a brief consideration of the optical conditions 

 in the case would show that it can only be accomplished in 

 a few ways. Not only does the passage of the light directly 

 tend to set up molecular rearrangements in the refracting 

 matter which lies before the retina, but out of those rearrange- 

 ments there are very few which can assist the focalizing pro- 

 cess, so that natural selection, in preserving the best-refracting 

 eyes, would have but very few directions in which to act. 

 The anterior membrane might differentiate into a number of 

 converging lenses, as in the higher annulosa, but if such a 

 differentiation did not occur, it is difficult to see how the 

 needful refraction could be secured, save by the differentiation 

 of the successive strata which we call the aqueous, crystalline, 

 and vitreous humours. This may serve to indicate the course 

 of explanation to be taken. The physical conditions for 

 securing very efficient vision being thus limited, and direct 

 adaptation being such an important factor in the process, 

 it does not seem at all strange that two eyes quite similar in 

 structure should be independently produced. A precisely 

 similar argument will apply to the case of the ear. And the 

 force of these considerations is still further increased when 

 we learn from Prof. Gegenbaur that the resemblances be- 

 tween the eyes of vertebrates and the eyes of cuttle-fishes are 

 only superficial analogies, and not fundamental homologies, 

 as Mr. Mivart's very exaggerated statement might lead one 

 to suppose. 



