366 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



ized and modern type of Reptilia, the segmentation te 

 complete. 



The segmentation of the limbs in the Vertebrata is 

 a simple mechanical problem. Paleontology and em- 

 bryology concur in proving that the limbs originated 

 in primitive folds in the external integument, and that 

 their connection with the internal skeleton was of later 

 accomplishment, has been shown by Wiedersheim. 

 At first free, they sought points of support on the 

 skeleton, but did not lose their free mobility when 

 this contact was attained. Appropriately to the me- 

 chanical conditions of rigidity and flexibility neces- 

 sary to their use in a fluid medium, they were orig- 

 inally composed of slender rods which were segmented 

 by interruptions at suitable points. The articulations 

 of the fin-rays of fishes have been made the subject 

 of an interesting research by Ryder, who finds them 

 to be fractures, due to flexures during motion in the 

 water medium. 1 The limb of land vertebrates (the 

 chiropterygium) was derived from one of the forms of 

 fins (rhipidopterygium) of water vertebrates. This is 

 the simple type of primitive fin displayed by the Pale- 

 ozoic Teleostomi of the superorder Rhipidopterygia. 

 Whether the subdivisions of the chiropterygium, the 

 propodial, metapodial, and phalangeal bones, etc., 

 were divided from the primitive branches of the archi- 

 pterygium, as held by Gegenbaur, or whether they 

 have developed by sprouting from a simple axial series 

 of segments, as held by Baur, or whether, as I have 

 suggested, it is a derivation from the rhipidopterygian 

 type of paired fin, is not yet decided. In either case, 

 the limbs of the first land animals were segmented and 

 flexible at the joints between the segments. The ne- 



\Proceedings of the American Philosophical Society, 1889, p. 547. 



