388 ELEMENTARY GEOLOGY 
and on the average, the history of the present is merely 
a continuation of the past. Of this there are so many 
evidences, that it is all but a universally accepted prin- 
ciple in geology. 
History involves stages of progression. For any 
adequate understanding of the earth’s past, we must 
seek to recognize such stages. Indeed, until the early 
part of this century, when a means was found of dis- 
tinguishing the ancient from the medizval and modern 
periods of the earth’s history, the science of geology 
cannot be said to have really had its birth. To William 
Smith, an English surveyor, belongs the honor of 
first finding this clue. He discovered that animal re- 
mains in the rocks, or fossils, furnish a means of deter- 
mining how ancient the strata are; and from that 
time to the present, the study of these remains has 
been one of the most important features of geology. 
The Fossil. — The name /ossi/' is now applied to any 
organic remains, preserved in any natural deposit in or 
on the earth’s crust, whether this be an animal or 
plant, or any part of such organisms. So it may be 
either an entire animal or plant, a fragment, an im- 
pression, or a cast of one. It may be the leaf or seed 
of a tree, or its bark or root; or it may be the shell 
of a clam, the tooth of a shark, or merely a print left 
in the rock by either of these. 
1 Originally meaning anything dug up, either mineral or animal. 
