5 4 EVOLUTION, GENETICS, AND EUGENICS 



same typical structure to the performance of widely different func- 

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

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

 therefore admit of being equally well explained by descent from a 

 common ancestry; while all cases of widely different structures per- 

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

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

 therefore suggestive of independent variations arising in the different 

 lines of hereditary descent. 



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

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

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

 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 resem- 

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

 thesis 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 

 typical structures modified so as to perform very different functions 

 we never find any of these particular types of structure in other large 

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



