THE PERIOD OF INCUBATION 77 



All through the middle decades of the century 

 Darwin continued to labour at his vast accumulation of 

 illustrative facts ; and side by side with his continuous 

 toil, outside opinion kept paving the way for the final 

 acceptance of his lucid ideas. The public was buying 

 and reading all the time its ten editions of the ' Vestiges 

 of Creation.' It was slowly digesting Lyell's ' Principles 

 of Geology,' in which the old cataclysmic theories were 

 featly demolished, and the uniformitarian conception 

 of a past gradually and insensibly merging into the 

 present was conclusively established. It was getting 

 accustomed to statements like those of the younger 

 St. Hilaire, in 1850, that specific characters may be 

 modified by changes in the environing conditions, and 

 that the modifications thus produced may often be of 

 generic value may make a difference so great that we 

 must regard the product not merely as belonging to a 

 distinct species, but even to a distinct genus or higher 

 kind. In 1852 Herbert Spencer published in the 

 ' Leader ' his remarkable essay, contrasting the theories 

 of creation and evolution, as applied to organic beings, 

 with all the biting force of his profound intelligence ; 

 and in 1855, the same encyclopaedic philosopher put 

 forth the first rough sketch of his 'Principles of 

 Psychology,' in which he took the lead in treating the 

 phenomena of mind from the point of view of gradual 

 development. In that extraordinary work, the philo- 

 sopher of evolution traced the origin of all mental 

 powers and faculties by slow gradations from the very 

 simplest subjective elements. The 'Principles of 

 Psychology ' preceded the ' Origin of Species ' by nearly 

 five years ; the first collected volume of Mr. Spencer's 



