X OBITUARY 301 



for far more important researches. Psychology 

 will be based on a new foundation, that of the 

 necessary acquirement of each mental power and 

 capacity by gradation. Light will be thrown on 

 the origin of man and his history." It is one of 

 the curiosities of scientific literature, that, in the 

 face of this plain declaration, its author should have 

 been charged with concealing his opinions on the 

 subject of the origin of man. But he reserved the 

 full statement of his views until 1871, when the 

 " Descent of Man " was published. The " Expres- 

 sion of the Emotions " (originally intended to form 

 only a chapter in the " Descent of Man ") grew into 

 a separate volume, which appeared in 1872. 

 Although always taking a keen interest in geology, 

 Darwin naturally found no time disposable for 

 geological work, even had his health permitted it, 

 after he became seriously engaged with the great 

 problem of species. But the last of his labours is, 

 in some sense, a return to his earliest, inasmuch as 

 it is an expansion of a short paper read before the 

 Geological Society more than forty years before, 

 and, as he says, " revived old geological thoughts " 

 (I, p. 98). In fact, " The Formation of Vegetable 

 Mould through the Action of Worms," affords as 

 striking an example of the great results produced 

 by the long-continued operation of small causes as 

 even the author of the " Principles of Geology " 

 could have desired. 



In the early months of 1882 Darwin's health 



