PRESENT PROBLEMS 577 



crossopterygian fishes, fishes which are now represented only by the 

 bichir (Polypterus) of Africa. It is interesting to recall parenthetically 

 that two naturalists, Harrington, an American, and Budgett, an 

 Englishman, have given their lives to the solution of this problem 

 in searching for the embryology of Polypterus. The latter explorer 

 only was successful. 1 



Missing Links between the Great Classes of Vertebrates 



Among the varied fins of the crossopterygians we have nearly, but 

 not actually, discovered the prototype of the hand and the foot, the 

 fingers and toes, of the primordial amphibian. Volumes upon volumes 

 have been written by embryologists and comparative anatomists on 

 the hypothetical transformation of the fin into the hand. 2 Consider- 

 ing the supreme value of the hand and foot in vertebrate history, this 

 was certainly the most momentous transformation of all and worthy 

 of volumes of speculation; but as a matter of fact, the speculation has 

 been a total failure, and this problem of problems will only be settled 

 by the future discovery in Devonian rocks of the actual connecting 

 link, which will be a partly air-breathing fish, capable of emerging 

 upon land, in which the cartilages of the fin will be found disposed 

 very much as in the limbs of the earliest Carboniferous amphibians. 

 The unity of composition in the hand and the foot points to an original 

 similarity of habit in the use of these organs. 



This missing point of contact, or of the actual link between am- 

 phibians and fishes, is equally characteristic of paleontology as his- 

 tory from the top to the bottom of the animal scale. We are positive 

 that amphibians descended from fishes, 3 probably of the crossoptery- 

 gian kind, but the link still eludes us; we have brought the reptiles 

 within close reach of the amphibians, 4 but the direct link is still to be 

 found; mammals are in close proximity to a certain order of reptiles, 5 



1 J. Graham Kerr, The Budgett Memorial (1) Note on the Developmental 

 Material of Polypterus obtained by the late Mr. J. S. Budgett. Reports of British 

 Association for the Advancement of Science, Section D, Cambridge, 1904, p. 29. 



2 Wiedersheim [Parker, W. N.]. See the bibliography of Fins and Limbs in 

 Wiedersheim's Elements of the Comparative Anatomy of Vertebrates, translated by 

 W. N. Parker, 1897. See also Gill, Homologies of Anterior Limbs, Science, N. S. 

 xvii, March 27, 1903, p. 488. 



3 A. Smith Woodward, Outlines of Vertebrate Paleontology, Cambridge, 1898, 

 pp. 123-125. 



4 Baur, Case, Cope, Osborn, Broili, F. See various papers on the Cotylosauria 

 (Pareiasauria) by Baur, Case, Cope. For a summary of their relations see Osborn, 

 The Reptilian Subclasses Diapsida and Synapsida, Memoirs, American Museum 

 of Natural History, vol. i, pt. vni, Nov. 1903, p. 456. Especially, Broili, Stammrep- 

 tilien, Anatomischer Anzeiger, xxv, No. 23, 1904, pp. 577-587. 



5 H. F. Osborn. For a summary see The Anomodontia, Reptilian Subclasses 

 Diapsida and Synapsida, Memoirs, American Museum of Natural History, vol. 

 i, pt. vni, Nov. 1903, pp. 460-466; The Origin of the Mammalia, American Natur- 

 alist, vol. xxxn, May, 1898, pp. 309-334. See various papers on the Anomodontia 

 by Owen, Seeley, Cope, Baur. 



" R. Broom. [Many recent papers by Broom on the Theriodontia and their 



