III.] INDEPENDENT SIMILARITIES OF STRUCTURE. 91 



Some may be disposed to object that the conditions 

 requisite for effecting vision are so rigid that similar results 

 in all cases must be independently arrived at. But to this 

 objection it may well be replied that Nature herself has 

 demonstrated that there is no such necessity as to the de- 

 tails of the process. For in the higher Annulosa, such as 

 the dragon-fly, we meet with an eye of an unquestionably 

 very high degree of efficiency, but formed on a type of 

 structure only remotely comparable with that of the fish 

 or the cephalopod. The last-named animal might have had 

 an eye as efficient as that of a vertebrate, but formed on a 

 distinct type, instead of being another edition, as it were, 

 of the very same structure. 



In the beginning of this chapter examples have been 

 given of the very diverse mode in which similar results 

 have in many instances been arrived at; on the other hand, 

 we have in the fish and the cephalopod not only the eye, 

 but at one and the same time the ear also similarly evolved, 

 yet with complete independence. 



Thus it is here contended that the similar and complex 

 structures of both the highest organs of sense, as developed 

 in the vertebrates on the one hand, and in the mollusks on 

 the other, present us with residuary phenomena for which 

 " Natural Selection " alone is quite incompetent to account ; 

 and that these same phenomena must therefore be consid- 

 ered as conclusive evidence for the action of some other 

 natural law or laws conditioning the simultaneous and in- 

 dependent evolution of these harmonious and concordant 

 adaptations. 



Provided with this evidence, it may be now profitable 

 to enumerate other correspondences, which are not perhaps 

 in themselves inexplicable by Natural Selection, but which 

 are more readily to be explained by the action of the un- 

 known law or laws referred to which action, as its neces- 

 sity has been demonstrated in one case, becomes a priori 

 probable in the others. 



