PHYSICS AND MATHEMATICS TO GEOLOGY. 153 
ean fully grasp, or for a scientist who believes the earth created for the 
special purpose of supplying problems of precisely that amount of diffi- 
culty which he personally is able to solve, but from a common-sense 
point of view it seems utterly irrational. No physicist or geologist has 
any reason to suppose that there are not numerous problems whose full 
comprehension requires more extensive knowledge than is possessed by 
himself or any of his contemporaries. 
The necessity for theories has been eloquently urged by Prof. Dar- 
win,* who says: *‘ A theory is, then, a necessity for the advance of sci- 
ence, and we may regard it as the branch of a living tree, of which 
facts are the nourishment.” Employing this simile, | must confess that 
the subject treated in this paper resembles, in my opinion, a tree which 
combines a sad deficiency of sap with a great superfluity of branches. 
It will I dare say be generally admitted that the premature craving 
after a finality of knowledge has been responsible for numerous fruit- 
less speculations in the past, and it seems only too probable that the 
impatience of the mind with its own ignorance is the principal founda- 
tion of much of the theory of to-day. The satisfaction derived from the 
contemplation of simple and comprehensive laws may suffice perhaps 
to prove that the powers of the mind are limited, but hardly that the 
processes of nature are simple. 
* Nature, 1886, vol. Xxxtv, p. 420, Address to British Association, section A. 
