LECT. in Geological Succession 95 



ever been a craving to explain the origin of things. But 

 these older interpretations of nature were rather exercises 

 of the imagination than results of observation and de- 

 duction. 



In the gradual growth of knowledge regarding the 

 history of our globe, it is surprising how late men were 

 in realising that this knowledge must be based not on 

 mere speculation, but on patient investigation of what 

 evidence can be gathered from the structure of the planet 

 itself. Slowly and laboriously the truth was reached that 

 the rocks which form the terrestrial crust bear witness to 

 the passage, not of one or two, but of a whole series of 

 revolutions, that these changes occupied vast intervals of 

 time, and that while they varied indefinitely in their local 

 effects from one region to another, they were but incidents 

 in one vast onward march of development which embraced 

 the whole globe within its influence. What we now know 

 as the doctrine of geological succession, in other words, 

 the history of the evolution of the earth, during a prolonged 

 series of ages up to the present time, took shape with 

 extreme slowness, each generation adding a little to the 

 basis of fact and to the superstructure of inference. 



There were in especial two lines of investigation along 

 which progress could be made. On one of these, the various 

 masses of rock that are visible over the surface of the globe 

 had to be studied with a view to the determination of their 

 origin and sequence. On the other line, the details of these 

 rock -masses, and more particularly of the sedimentary 

 series, had to be worked out, and their organic contents 

 to be noted, in order to ascertain how far the living 

 creatures of older times differed from those of the present. 



