EXTINCT ANIMALS 



The estimate thus given probably does not 

 fully represent the time which has elapsed. If 

 you take a thousand years for each foot, you 

 only get an approximate measure of the time 

 represented, because a great deal more time has 

 passed than is actually shown by the permanent 

 deposits or strata. Strata have been broken 

 up by the sea and water, and have been deposited 

 again and again ; and it is probable that a 

 much longer time has elapsed than one thousand 

 years for each foot of the deposits which form 

 the stratified crust of the earth. 



An important general fact, which I cannot 

 dwell on further, is that whilst it is true that the 

 great animals occur in the later stage of the 

 world's history, there is a gradual succession 

 from simpler to more complex forms of life. 

 We get fishes at the top of the Silurian : and 

 we get In the Carboniferous great amphibians ; 

 and the first reptiles in the Permian ; and then 

 we get birds and crocodiles in the Triassic ; 

 and the first hairy warm-blooded quadrupeds 

 in the Jurassic. Thus the different kinds of 

 animals succeed one another in the order of 

 increasing complexity of structure so that the 

 highest animals are the latest to appear. 



62 



