EVIDENCES FROM MORPHOLOGY 135 



;ame typical structure to the performance of widely different func- 

 ions — or cases of homology without analogy — are cases which come 

 vithin the limits of the same natural group of plants and animals, and 

 herefore admit of being equally well explained by descent from a 

 ■ommon ancestry; while all cases of widely different structures per- 

 orming the same function — or cases of analogy without homology, 

 ire to be found in different groups of plants or animals, and are 

 herefore suggestive of independent variations arising in the different 

 ines of hereditary descent. 



To take a specific illustration. The octopus, or devil-fish, belongs 

 o a widely different class of animals from a true fish; and yet its eye, 

 n general appearance, looks wonderfully like the eye of a true fish. 

 \^ow, Mr. Mivart pointed to this fact as a great difficulty in the way 

 )f the theory of evolution by natural selection, because it must clearly 

 )e a most improbable thing that so complicated a structure as the eye 

 )f a fish should happen to be arrived at through each of two totally 

 lifferent lines of descent. And this difficulty would, indeed, be a 

 ormidable one to the theory of evolution, if the similarity were not 

 )nly analogical but homological. Unfortunately for the objection, 

 lowever, Darwin clearly showed in his reply that in no one anatomical 

 )r homologous feature do the two structures resemble one another; 

 ;o that, in point of fact, the two organs do not resemble one another 

 n any particular further than it is necessary that they should, if both 

 ire to be analogous, or to serve the same function as organs of sight. 

 3ut now, suppose that this had not been the case, and that the two 

 structures, besides presenting the necessary superficial or analogical 

 •esemblance, had also presented an anatomical or homologous resem- 

 jlance, with what force might it have then been urged, — your hypo- 

 hesis of hereditary descent with progressive modification being here 

 excluded by the fact that the animals compared belong to two widely 

 lifferent branches of the tree of life, how are we to explain the identity 

 if type manifested by these two complicated organs of vision ? The \ 

 m\y hypothesis open to us is intelligent adherence to an ideal plan or » 

 nechanism. But as this cannot now be urged in any comparable 

 :ase throughout the whole organic world, we may, on the other hand, 

 :)resent it as a most significant fact, that while within the limits of the 

 -ame large branch of the tree of life we constantly find the same 

 ypical structures modified so as to perform very different functions 

 .ve never find any of these particular types of structure in other large 

 jranches of the tree. That is to say, we never find typical structures 



