THE ENDOSKELETON 187 



digit upon the inner, and three upon the outer side to serve as 

 lateral rays. Aside from the unsafe basis, however, upon 

 which all such theories are founded, their use as an argument 

 is nullified by the ease with which, in turn, each digit, with its 

 associated bones, may serve either as a hypothetical central 

 axis or as a lateral ray. 



As opposed to the theories thus far recorded, which seek 

 to derive the entire free limb from the fin, is a more recent 

 one, based upon the anterior fin of the ganoid Polypterus, in 

 which ulna and radius are derived from the fin, while the 

 humerus represents an element to be detached later from the 

 shoulder-girdle. In this fin (Fig. 51, ft), the pro- and meta- 

 pterygium, already in the form of long bones, articulate with 

 a projecting point of the girdle, while the meso-pterygium, 

 reduced to a small, disc-shaped element, lies between them 

 but detached from both. // now the long pro- and meta-ptery-\ 

 gia are taken as radius and ulna respectively, ignoring tfie 

 meso-pterygium for the present, the projecting process of the 

 shoulder-girdle, which articulates with both, and which might 

 easily become detached from the main mass if of functional\ 

 advantage, would become a humerus. The mesopterygium 1 

 would thus become intermedium or intermedium and centrale, \ 

 leaving the remaining parts of the carpus, the metacarpus and 

 phalanges, to be formed from fin-rays. This is in many ways 

 the most satisfactory solution thus far, and the fact that it 

 rests upon the condition found in a single species is no real 

 objection, since it is altogether probable that terrestrial verte- 

 brates were originally derived from a single form, perhaps 

 a single species, and that Polypterus, with its close anatomical 

 correspondence with the lowest urodelous amphibia, is nearly 

 allied to that form. A more serious objection lies in the fact 

 that the posterior fin cannot be as easily developed into a 

 cheiropterygium as can the anterior one ; still the anterior and 

 posterior limbs may not have had exactly the same origin or 

 _^ early history, since a later similarity of use \vould cause a 

 convergence in anatomical structure. Moreover, as a mat- 

 ter of fact, the pelvic fin of Polypterus exhibits proximally 



