150 THE MEANING OF EVOLUTION 
far, but must surely be measured in tens of millions 
of years. 
When we attempt to study the past we find its vari- 
ous epochs unequally clear to us. In human history 
only quite modern times are absolutely clear. The 
history of the Middle Ages is distinct enough for us 
to build for ourselves a picture of the time with rea- 
sonable hope of gaining a correct view of the state of 
affairs. Back of this comes the long stretch of the 
Dark Ages, in which here and there we have bright 
spots, but it will perhaps long be impossible to portray 
clearly the life of the people. Getting back to the 
Romans, things once more become reasonably plain, 
as is true also in the case of Greek history. Back of 
this stretches the Egyptian with fair precision, and, 
older than it, the Babylonian and Chaldean. But these 
past three have not left nearly so definite an account 
for us as did the later civilizations of Greece and 
Rome. 
When we try to go back of these we must change 
our method of study entirely. Writing is absent, and 
all we know of earlier men must be inferred from a 
few pictures that were daubed on the rocks or carved 
in ivory or bone, from tools made of stone or bone, 
from a few metal or stone ornaments, or from the 
bones of the men themselves. Even so, the history 
fades out without telling us its own beginnings. It 
