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 
2 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 
