526 Edward Livingston Youmans. 



interpretation of mental phenomena upon the basis of Evo- 

 lution. We have seen that two years after the publica- 

 tion of the Psychology, or in 1857, Mr. Spencer had ar- 

 rived at the law of Evolution as a universal principle of 

 Nature, and worked it out both inductively as a process of 

 increasing heterogeneity, and deductively from the princi- 

 ples of the instability of the homogeneous and the multi- 

 plication of effects. How far Mr. Spencer was here in ad- 

 vance of all other workers in this field, will appear when 

 we consider that the doctrine of Evolution, as it now 

 stands, was thus, in its universality, and in its chief out- 

 lines, announced by him two years before the appearance of 

 Mr. Darwin's Origin of Species. 



A principle of natural changes more universal than any 

 other known, applicable to all orders of phenomena, and so 

 deep as to involve the very origin of things, having thus 

 been established, the final step remained to be taken, which 

 was to give it the same ruling place in the world of thought 

 and of knowledge that it has in the world of fact and of 

 Nature. A principle running through all spheres of phe- 

 nomena must have the highest value for determining scien- 

 tific relations ; and a genetic law of natural things must 

 necessarily form the deepest root of the philosophy of nat- 

 ural things. It was in 1858, as Mr. Spencer informs me, 

 while writing the article on the Nebular Hypothesis, that 

 the doctrine of Evolution presented itself as the basis of a 

 general system under which all orders of concrete phenom- 

 ena should be generalized. Already the conception had 

 been traced out in its applications to astronomy, geology, 

 biology, psychology, as well as all the various superor- 

 ganic products of social activity ; and it began to appear 

 both possible and necessary that all these various concrete 

 sciences should be dealt with in detail from the Evolution 

 point of view. By such treatment, and by that only, did 

 it appear practicable to bring them into relation so as to 



