1885.] Progressive and Retrogressive. 143 
be determined by the discovery and investigation of the ancestors 
themselves, as they are preserved in the crust of the earth. In 
all cases this discovery confirms and establishes such definite con- 
clusions as may be derived from embryology. It is also clear 
that on the discovery of phylogenetic series it becomes at once 
possible to determine the nature of defective types. It becomes 
possible to ascertain whether their rudimental parts represent the 
beginnings of organs, or whether they are the result of a process 
of degeneration of organs once well developed. 
A great deal of light has been happily thrown on this question 
as regards the “Vertebrata, by the recent work done in North 
American paleontology. The lines of descent of many of the 
minor groups have been positively determined, and the phylogen- 
etic connections of most of the primary divisions or classes have 
been made out. The result of these investigations has been to 
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 retro- 
gressive. 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? 
Omitting from consideration the two classes above entio et 
whose remains have not yet been certainly found in a fossil state, 
t See Origin of Genera, E. D. Cope, Philadelphia, 1868, where these terms are 
introduced. 
. See Der Ursprung der Wirbelthiere u. d. Princip des Functionwechsels, Leipsic, 
1875. 
*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 bird, the reptiles and the true fishes, or Hyopomata, including the spe- 
cial phylogenies of the Batrachia and Reptilia, and some of the Mammalia. See the 
following references: AMERICAN NATURALIST, 1884, p. 1136; Proceedings Acad- 
emy Philadelphia, 1867, p. 234; Proceedings American ‘Philosoph. Society, 1884, p. 
585; AMERICAN NATURALIST, 1884, p. 27; Proceedings American Association for 
the Advancement of Science, XIX, 1871, p. 233; Proceedings American Philosophical 
Society, 1882, p. 447; AMERICAN NATURALIST, 1884, pp. 261 and 1121; Report 
U. S. Geol. Survey W. of 1ooth Mer., G. M. Wheeler, 1877, IV, 1, p. 282. ; 
