EVOLUTION OF THE VERTEBRATA. 317 



prove that the evolution of the Vertebrata has proceeded not only 

 on lines of acceleration, but, to a much greater extent than has 

 been heretofore suspected, on lines of retardation.* That is, that 

 evolution has been not only progressive, but at times retrogressive. 

 This is entirely in accord with the views derived by Dohrn from 

 embryology,! who, however, wrote only of the origin of the 

 Vertebrata as a whole and not of its divisions, excepting only 

 the Leptocardii and Marsipobranchii, that is, of the sand-lance 

 and the lampreys and hags. The demonstration of such relations 

 for the higher Vertebrata is now done nearly for the first time. J 



Omitting from consideration the two classes above mentioned, 

 whose remains have not yet been certainly found in a fossil state, 

 there remain the following : the Pisces, Batrachia, Eeptilia, Aves, 

 and Mammalia. 



The Mammalia have been traced to the theromorphous reptiles 

 through the Monotremata. The birds, some of them at least, ap- 

 pear to have been derived from the Dinosaurian reptiles. The rep- 

 tiles, in their primary representative order, the Theromorpha, have 

 been probably derived from the rhachitomous Batrachia.* The 

 Batrachia have originated from the subclass of fishes, the Dipnoi, 

 though not from any known form. I have shown that the true 

 fishes or Teleostomi have descended from an order of sharks, || 

 the Ichthyotomi, which possess characters of the Dipnoi also. 

 The origin of the sharks remains entirely obscure, as does also 

 that of the Pisces as a whole. Dohrn believes the Marsii^o- 



* See "Origin of Genera," E. D. Cope, Philadelphia, 1868, where these terms 

 are introduced. 



f See "Der Ursprung der Wirbelthiere u. d. Princip des Functionwechsels," 

 Leipsic, 18'75. 



^ "On the Phylogeny of the Vertebrata," Cope, " Amer. Naturalist," Dec., 1884. 

 I here remark that my researches have now, as I believe, disclosed the ancestry of 

 the mammals, the birds, the reptiles, and the true fishes, or Teleostomi, including 

 the special phylogenies of the Batrachia and Reptilia, and some of the Mammalia. 

 See the following references: "American Naturalist," 1884, p. 1136 ; "Proceedings 

 Academy Philadelphia," 186Y, p. 234; "Proceedings American Philosoph. Society," 

 1884, p. 585; "American Naturalist," 1884, p. 27; "Proceedings American Asso- 

 ciation for the Advancement of Science," xix, ISTl, p. 233; "Proceedings Amer- 

 ican Philosophical Society," 1882, p. 447; "American Naturalist," 1884, pp. 261 

 and 1121 ; "Report U. S. Geol. Survey W. of lOOth Mer.," G. M. Wheeler, 1877, 

 iv, ii, p. 282. 



* Through the batrachian order Embolomeri. (Ed. 1886.) 

 I "Proceedings Am. Phil. Soc," 1884, p. 585. 



