THE BEGINNINGS OF IIFE. 62 1 



induced by the slow accumulation of minute differ- 

 ences ^ So that if the descendants of similar organisms 

 (derived perhaps from totally independent stocks) have 

 been exposed to notably different external conditions 

 in different ages j or if in any of them modifications 

 have otherwise arisen, the forms ultimately produced 

 along such lines of development may be widely dif- 

 ferent from one another, although belonging to similar 

 types. 



And at different periods in the earth's history, 

 specializations, now of one type and now of another_, 

 have been more and more manifest or dominant. In 

 the Silurian epoch, strange crustacean Trilobites 

 abounded in all the seas. In the Devonian epoch 

 fishes of a remarkable structure were most plentifully 

 represented ; whilst the same may be said of the cup- 

 like Encrinites in the earlier Carboniferous period, 

 though in the later portions of this epoch they were 

 altogether thrown into the shade by that vast tropical 

 vegetation from which we now derive our supplies of 

 coal. In the Oolitic period, or so-called <^age of rep- 

 tiles,'' we have a most remarkable abundance of Saurian 

 forms, and the Amphibian type reached its highest de- 

 velopment. Huge Ichthyosauri and Plesiosauri swam 

 in the lakes and rivers, whilst strange and gigantic 



* As Mr. Darwin remarks (loc. cit., p. 570): — 'The mind cannot 

 possibly grasp the full meaning of the term of even ten million years : 

 it cannot add up and perceive the full effects of slight variations accumu- 

 lating during an almost infinite number of generations.' 



