58 Darwin^ and after Darwin. 



Now, Mr. Mivart pointed to this fact as a great 

 difficulty in the way of the theory of evolution by 

 natural selection, because it must clearly be a most 

 improbable thing that so complicated a structure as 

 the eye of a fish should happen to be arrived at 

 through each of two totally different lines of descent. 

 And this difficulty would, indeed, be a formidable one 

 to the theory of evolution, if the similarity were not 

 only analogical but homological. Unfortunately for 

 the objection, however, Darwin clearly showed in his 

 reply that in no one anatomical or homologous 

 feature do the two structures resemble one another ; 

 so that, in point of fact, the two organs do not 

 resemble one another in any particular further than it 

 is necessary that they should, if both are to be 

 analogous, or to serve the same function as organs of 

 sight. But now, suppose that this had not been the 

 case, and that the two structures, besides presenting 

 the necessary superficial or analogical resemblance, 

 had also presented an anatomical or homologous 

 resemblance, with what force might it have then been 

 urged, Your hypothesis of hereditary descent with 

 progressive modification being here excluded by the 

 fact that the animals compared belong to two widely 

 different branches of the tree of life, how are we to 

 explain the identity of type manifested by these two 

 complicated organs of vision ? the only hypothesis 

 open to us is intelligent adherence to an ideal plan or 

 mechanism. But as this cannot now be urged in any 

 comparable case throughout the whole organic world, 

 we may on the other hand present it as a most significant 

 fact, that while within the limits of the same large 

 branch of the tree of life we constantly find the same 



