EXCURSIONS OF AN EVOLUTIONIST 



we see more clearly into the details of life at 

 that time, and consequently have a more vivid 

 picture of it before us ; and this more vivid 

 picture, as is natural, usurps an undue place in 

 our minds. 



The force of these remarks will be obvious 

 when it is stated that in point of fact the begin- 

 ning of the Tertiary period carries us back barely 

 one twentieth -part of the way toward the first 

 beginnings of fossil-bearing strata. In the table 

 that follows, I have tried to give something like 

 a just idea of the relative lengths of geological 

 epochs, in accordance with the views now gener- 

 ally adopted by geologists. Let us first suppose 

 the entire lapse of time since the oldest Lauren- 

 tian strata began to be deposited, down to the 

 present day, to be divided into ten equal periods, 

 or aeons, such as I have marked off" on the table 

 with dotted lines. Then the Laurentian epoch 

 fills three of these great aeons, to begin with. 

 Here we find (with the exception of the Cana- 

 dian eozoon, the organic nature of which has 

 been disputed) only indirect traces of life, such 

 as limestone, which probably came from shells. 

 But, remembering how soft and perishable are 

 all the lowest organisms, and remembering how 

 considerably these oldest rocks have been af- 

 fected by volcanic heat, we need not be sur- 

 prised at finding the records of life in them 

 very scanty and obscure. Next, the Cambrian 

 4 



