PREFACE. ix 



by Spencer in his " Principles of Biology " as respects the effect of motion 

 on structure in general,* and in the particular case of the origin of verte- 

 brsB.t Fifth, the complementary development of parts had also been 

 pointed out by Herbert Spencer,! 



Seventh, tlie explanation of inheritance by the transmission of the type 

 of growth-force possessed by one generation to another. This doctrine was 

 subsequently announced by Haeckel, under the name of perigenesis,^ 

 and is the only good hypothesis yet proposed for the explanation of this 

 phenomenon. Eighth, the theory of "intelligent selection,"- or the agency 

 of the intelligence of a living being in directing its movements, and there- 

 fore its growth, although a plain and necessary consequence of the " law of 

 use and effort," had not been, so far as I am aware, announced prior to 

 the publication of this paper. This important theory at once opened the 

 way for an investigation of the general relations of mind to evolution, which 

 involved the question of the origin and development of mind itself. These 

 questions were more fully discussed in the papers of Part IV, on " Metaphys- 

 ical Evolution." Ninth, the origin of mimetic analogy. The explanation 

 offered is almost necessary, if the doctrine of the influence of effort on struct- 

 ural growth be true. 



I. Evolution and its Consequences, From the *' Penn Monthly Maga- 

 zine," Philadelphia, for May, July, and August, 1872, 



This is a popular exposition, with elaboration, of the doctrines contained 

 in the preceding essays. The evolution of mind is more fully stated, the 

 hypothesis adopted being that proposed by Spencer in his " Principles of 

 Psychology," published in 1855. 



VII. The Homologies and Origin of the Types of Molar Teeth 

 OF THE Mammalia Edtjcabilia. From the " Journal of the Academy of 

 Natural Sciences," Philadelphia, March 30, 1874. 



The object of this paper was to show that the various more or less com- 

 plex types of molar "teeth displayed by the Ungulate Mammalia are referable 

 to modifications of a primitive quadituberciilar type, from which they were 

 supposed to have been descended. The histories and homologies of the 

 carnivorous dentitioa were not included in this paper. In order to com- 

 plete the subject, I have inserted brief notes of the conclusions I have since 

 attained in this field : first, as to the type of inferior sectorial teeth, in 1875 ; 

 and, second, as to the superior molars, in 1883. 



At the end of the paper a similar- comparison between the feet of the 

 same type of mammals is made, and general conclusions reached in the fol- 

 lowing language : " I trust that I have made it sufiiciently obvious that the 

 primitive genera of this division of mammals [Mammalia Educabilia = Un- 

 guiculata and Ungulata sensu lata] must have been bunodonts with penta- 

 ■daetyl plantigrade feet." 



Vol. ii, p. 167. t Vol. ii, p. 195. X " Principles of Biology." 



* See Ryder, "American Naturalist," January, 1879. 



