INTRODUCTION 7 
great principle in geology: that im the present we have 
an ilustration of what has happened in the past. In 
human history we assume that, in general, our ancestors 
have been guided by the same motives which actuate us. 
There have always been evil and good, sloth and ambi- 
tion, weakness and strength, and the other features of 
mind and body upon which the progress of humanity 
depends. We know that these rule the world to-day, 
and that they have been the main factors in past 
development, as revealed by history; we assume that 
the same conditions existed before history was recorded. 
So, while history has been complex, and many different 
results have been reached, there has always been at 
work the same set of conditions. 
The geologist, at first against’ his preconceptions, but 
now freely, sees in the pages of the earth’s history, 
positive proof that the changes which are now slowly 
operating on the surface of the earth have always 
been at work, and that the present and past are alike 
in this respect. This does not mean that nothing 
has ever been different; but that the processes of 
nature have remained essentially the same. There 
have been many complexities, but still in the present 
we see the conditions of the past. The science of 
geology is built upon this conception, whose proof is 
so abundant as to have won full acceptation from all 
modern workers in the subject. 
