MODERN EVOLUTION. 



189 



The Principles of Psychology was both preceded 

 and followed by a series of essays in which the 

 process of change from the " homogeneous to the 

 heterogeneous," i. e., from the seeming like to the 

 actual unhke, was expounded. Mr. Spencer tells 

 us that in 1852 he first became acquainted with 

 Von Baer's Law of Development, or the changes 

 undergone in each living thing, from the general to 

 the special, during its advance from the embryonic 

 to the fully-formed state. That law confirmed the 

 prevision indicated in the passages quoted above 

 from Social Statics, and impressed him as one of 

 the three doctrines which are indispensable elements 

 of the general theory of Evolution. The other two 

 are the Correlation of the Physical Forces, or the 

 transformation of diflerent modes of motion into 

 other modes of motion, as of heat or light into 

 electricity, and so forth, in Proteus-like fashion; and 

 the Conservation of Energy, or the indestructibility 

 of matter and motion, whatever changes or trans- 

 formations these may undergo. 



In permitting the quotation of the useful abstract 

 of the Synthetic Philosophy which, originally drawn 

 up for the late Professor Youmans, was imbodied 

 in a letter to the Athenaeum of 22d of July, 1882, Mr. 

 Spencer was good enough to volunteer the following 

 details to the writer: — 



" You are probably aware that the conception set 

 forth in that abstract was reached by slow steps dur- 

 ing many years. These steps occurred as follows: — 



