PREFACE. vii 



to a demonstration. Of the truth of the doctrine the author is more 

 than ever convinced, and he believes that paleontological discovery has 

 demonstrated it in many instances, and that other demonstrations will fol- 

 low. The fourth proposition (that of homologous groups) is now held 

 as a hypothesis explaining the phylogeny of various groups of animals. 

 For the descent of one homologous group from another, the term poly- 

 phyletic has been coined. It remains to be seen whether the doctrine 

 is of universal application or not. That homologous groups belong to 

 different geological horizons, as stated under the fifth head, has been 

 frequently demonstrated since the publication of the essay. That the sixth 

 proposition is true in a certain number of cases is well known, and it 

 follows that the seventh proposition is also true in those cases. The 

 latter hypothesis, which was originally advanced by Prof. Agassiz, is, how- 

 ever, only partially true, and the advance of paleontological study has not 

 demonstrated that it has had a very wide application in geological time. 



A proposition which was made prominent in this essay was, that the 

 prevalence of non-adaptive characters, in animals, proves the inadequacy of 

 hypotheses which ascribe the survival of types to their superior adaptation 

 to their environment. Numerous facts of this kind undoubtedly indicate 

 little or no activity of a selective agency in nature, and do point to the 

 existence of an especial developmental force acting by a direct influence on 

 growth. The action of this force is the acceleration and retardation ap- 

 pealed to in this paper. The force itself was not distinguished until the 

 publication of the essay entitled " The Method of Creation " (No. V), where 

 it was named growth-force, or bathmism. The energetic action of this force 

 accounts for the origin of characters, whether adaptive or non-adaptive, the 

 former diftering from the latter in an intelligent direction, which adapts 

 them to the environment. The numerous adaptive characters of animals 

 had by that time engaged the attention of the author, and he found that they 

 are even more numerous than the non-adaptive. Some of the latter were ac- 

 counted for on the theory of the " complementary location of growth-force." 



IV. The Hypothesis of Evolution, Phtsioal and Metaphysical. 

 " Lippincott's Magazine," Philadelphia, 1870 ; reprinted by Charles C. Chat- 

 field & Co. New Haven, 1870. 



This essay embraces a popular exposition of the principles maintained in 

 the essay entitled the " Origin of Genera," with some conclusions derived 

 from the general facts of anthropology. To this were added some facts in 

 the evolution of human physiognomy and human character, which had not 

 been previously thrown into harmony witli the laws already set forth. Un- 

 fortunately, the author attempted to correlate these again with the theories 

 of some theologians, and, in some instances, without success. A few 

 paragraphs have been stricken from this part of the essay, and others are 

 allowed to remain as illustrations of far-reaching hypotheses resting on 

 little information. 



V. The Method of Creation of Organic Types. From the "Pro- 

 ceedings of the American Philosophical Society," December, 1871; repub- 



