THE USES OF FOSSILS 395 
first and the second series of strata. If there were no 
fossils, we could gather only the fact of the break ; 
but finding remains in the two sets of strata, we can 
determine the geological age of each, and thus gain 
some idea of the lapse of time occupied by the dry- 
land condition. It may have been long enough for the 
animals to entirely change in kind. 
Evolution. As has been stated, one of the aithe 
tions made by these organisms is that of change, 
progress, and development. From the study of fossils, 
the doctrine of evolution has received great support. 
With the change of conditions, forms of animal and 
plant life have varied, new species have been evolved, 
races have come and gone, and always there has been 
indicated a grand forward movement. The earth has 
slowly changed for the better. 
Chronology. Since there has been this progress in 
animal and plant life, it follows that the rocks of the 
past contain a different assemblage of fossils from those 
stored in more recent strata. Therefore, if we know 
the kinds of animals that lived at different periods, we 
may by means of these identify the rocks. If in any 
place we should have a series of strata, one above the 
other, representing deposits made from the dawn of 
time to the present day, there would be found in these 
a record of the most important changes in animal 
and plant life. The lowest strata would contain 
