72 THE EVOLUTIONIST AT LARGE. 



these early deposits have been terribly twisted 

 and contorted by subsequent convulsions of 

 the earth, and most of them have been melted 

 down by volcanic action ; so that we can tell 

 very little about their original state. Thus 

 the history of life opens for us, like most 

 other histories, with a period of uncertainty : 

 its origin is lost in the distant vistas of time. 

 Still, we know that there was such an early 

 period ; and from the thickness of the rocks 

 which represent it we may conjecture that it 

 spread over three out of the ten great aeons 

 into which I have roughly divided geological 

 time. Next comes the period known as the 

 Cambrian, and to it we may similarly assign 

 about two and a half aeons on like grounds. 

 The Cambrian epoch begins with a fair 

 sprinkling of the lower animals and plants, 

 presumably developed during the preceding 

 age ; but it shows no remains of fish or any 

 other vertebrates. To the Silurian, Devo- 

 nian, and Carboniferous periods we may 

 roughly allow an aeon and a fraction each : 



