LIFE IN THE PAST I5I 



is quite as impossible for history to write its origins 

 as it is for man, from his own knowledge, to descrilje 

 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- V 

 tury, has held almost undisputed sway among men 



