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, 
wemay 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 
