36 THE CASE AGAINST EVOLUTION 



each other. In like manner, though both are devoted to the 

 same function, there exists between the leg of a man and the 

 leg of a spider a fundamental disparity in structure. 



At times, specialization for the selfsame function involves 

 the emergence of a similar modification or uniform structural 

 adaptation from a substrate of basic dissimilarity. In these 

 instances of parallel modifications appearing on the surface 

 of divergent types, we have something more than mere func- 

 tional resemblance. Structure is likewise involved, albeit 

 superficially, in the modification which brings about this exter- 

 nal uniformity. In such cases, analogy is spoken of as con- 

 vergence, a phenomenon of which the mole and the mole-cricket 

 constitute a typical example. The burrowing legs of the insect 

 are, so far as outward appearance goes, the exact replica on a 

 smaller scale of those of the mole, though, fundamentally, 

 their structure is quite unlike, the mole being built on the 

 endoskeletal plan of the vertebrates, whereas the mole-cricket 

 is constructed on the exoskeletal plan characteristic of the 

 arthropods. Speaking of the first pair of legs of the mole- 

 cricket, Thomas Hunt Morgan says: "By their use the mole- 

 cricket makes a burrow near the surface of the ground, similar 

 to, but of course much smaller than, that made by the mole. 

 In both of these cases the adaptation is the more obvious, be- 

 cause, while the leg of the mole is formed on the same general 

 plan as that of other vertebrates, and the leg of the mole- 

 cricket has the same fundamental structure as that of other in- 

 sects, yet in both cases the details of structure and the general 

 proportions have been so altered that the leg is fitted for en- 

 tirely different purposes from those to which the legs of other 

 vertebrates and other insects are put." (Quoted by Dwight in 

 "Thoughts of a Catholic Anatomist," p. 235.) In the analogies 

 of convergence, therefore, we have the exact converse of the 

 phenomenon so often encountered in connection with homology. 

 The latter exhibits a contrast between basic identity and super- 

 ficial diversity, the former a contrast between superficial con- 

 vergence and fundamental divergence. 



