528 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



a constant alteration of the phyla in definite directions, 

 towards greater perfection, that is, towards greater 

 complexity." Accordingly Nageli describes his theory 

 of internal causes, as the "Principle of Improvement." 

 "Superficial reasoners," he says, "have pretended to 

 discover mysticism in this. But the principle is one 

 of a mechanical nature and constitutes the law of per- 

 sistence of motion in the field of organic evolution. 

 Once the motion of evolution is started it cannot cease, 

 but must persist in its original direction." 



For convenience of reference I give a list of pa- 

 pers by American authors on this subject. European 

 authors, beginning with Lamarck, and including Spen- 

 cer, have implicitly included in their theses the effects 

 of mechanical strains and impacts in developing the 

 structures of animals. Pick, Henke, Tornier, and 

 others have attempted an exact demonstration of the 

 manner in which these forms of mechanical energy 

 have produced the results. These attempts have great 

 merit as physiological studies, but they have not been 

 used by their authors as illustrations of specific evolu- 

 tion, owing to the fact that they have not carried their 

 researches into the field of paleontology. 



LIST OF PAPERS BY AMERICAN AUTHORS WHO HAVE 



CONTRIBUTED TO THE EVIDENCE 



USED IN THIS BOOK. 



1866. Hyatt, A. Memoirs of the Boston Society of Natural His- 

 tory, p. 203. On the Parallelism Between Stages in the 

 Individual and those in the Groups of Tetrabranchiata. 



1866. Cope, E. D. Transactions of the American Philosophical 

 Society, p. 398. On the Cyprinidae of Pennsylvania. 



1871. Cope, E. D. Proceedings of the American Philosophical 

 Society, p. 259. On the Method of Creation of Organic 

 Types; reprinted in Origin of the Fittest, 1887, pp. 210, 

 195. 199- 



