174 PIONEERS OF EVOLUTION PART 



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 different 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 embodied 

 in a letter to the Athen&um of 22nd July 1882, Mr. 

 Spencer was good enough to volunteer the following 

 details to the present writer: 



' You are probably aware that the conception set 

 forth in that abstract was reached by slow steps 

 during many years. These steps occurred as 

 follows : 



1850. Social Statics : especially chapter ' General 

 Considerations.' (Higher human Evo- 

 lution.) 



1852. March. ' Development Hypothesis,' in the 

 Leader. (Evolution of Species), vid. 

 ante, p. 1 1 1 . 



1852. April. 'Theory of Population, etc.,' in 

 Westminster Review. (Higher human 

 Evolution.) 

 1854. July. 'The Genesis of Science ' in British 



