PALAEONTOLOGY 81 



was not at all variable. Such are the Crinoidea, re- 

 presented to-day by Pentacrinus ; the Nautilidea, 

 represented by Nautilus; and the Khynchocephalia, 

 represented by Sphenodon. However, most of the 

 classes of animals and plants have undergone but 

 little change since their first appearance. In com- 

 paratively few cases has change been rapid ; but it is 

 these rapidly changing forms which seem so re- 

 markable in our eyes, and give the impression that 

 great change is more universal than it really is. 

 Except the large Lycopods and Crustaceans of the 

 newer Palaeozoic, the reptiles and birds of the 

 Mesozoic, and perhaps a few of the Eocene hoofed 

 mammals, there is nothing among extinct plants and 

 animals that would appeal to the untrained eye as 

 anything remarkable and unlike living plants and 

 animals. Our views on this subject are much ex- 

 aggerated, owing to the numerous drawings and 

 models that have been made of a few of the most 

 extraordinary of the animals ; and we forget that 

 they were only a few among a host of quite ordinary 

 beings. 



The majority of invertebrates developed slowly and 

 steadily from the first to the present day (e.g. Echi- 

 noidea, Pelecypoda, Gastropoda, Decapoda, and 

 Insects) ; but others rapidly attained a maximum, 

 and then declined (e.g. Brachiopoda, Trilobita, and 

 Crinoidea). It is the same with the extinction of 

 groups. Most perish slowly, as is illustrated by the 

 gradual change from Deutozoic to Mesozoic life ; but 

 at the close of the Cretaceous there was a rapid 

 extinction of cephalopoda and reptiles 1 ; a fact very 



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