LIFE IN THE PAST I51I 
is quite as impossible for history to write its origins 
as it is for man, from his own knowledge, to describe 
his birth. 
What is true of the human story is quite as true of 
that of the earth. Recent steps are very plain. We 
may read them with considerable confidence. As we 
go deeper into the rocks and find older fossils, the 
evidence becomes less certain. The animals differed 
enough from those of to-day for us to be less sure 
what they were like. As we keep on moving back- 
ward through time, and downward through the rocks, 
we find, after a while, strata in which there are evi- 
dences of life that existed long ago, but in which 
these traces are so altered that it is impossible to tell 
what sort of living things existed; we learn only that 
they were alive. Going back still further, these fade 
out. There is no knowing when the earth began; 
there is no knowing when life began upon the earth. 
It is not meant that men have not wondered, even 
reckoned carefully, as to how long ago each of these 
events occurred. Many speculations have proved en- 
tirely useless, a few remain as yet neither confirmed 
nor disproved, and of such we shall speak. 
For the last hundred years the theory of the earth’s 
origin suggested by the Marquis Pierre Simon De La 
Place, of France, near the end of the eighteenth cen- 
tury, has held almost undisputed sway among men 
