74 THE LESSON OF EVOLUTION 



the biological evolution, competition grew keener, 

 species more quickly succumbed to others better 

 adapted to the circumstances, their term of life grew 

 shorter, and their geographical distribution was, in 

 consequence, more limited. Thus the differences be- 

 tween the faunas and floras of distant countries, 

 hardly recognisable during the Palaeozoic era, gradu- 

 ally became more and more pronounced up to the 

 present day. 



The principal features of the evolution of marine 

 animals during the last half of geological time are the 

 steady increase in numbers and variety of the reef- 

 building corals, the sea-urchins (JSchinoidea), the 

 bivalve and univalve mollusca, the decapod crusta- 

 ceans, and the actinopterygian fishes ; with the simul- 

 taneous decline of the Crinoidea and the Brachwpoda ; 

 while the Cephalopoda maintained a very important 

 position up to the close of the Mesozoic era, and then 

 collapsed. 



The subject naturally divides itself into three parts 

 Deutozoic, Mesozoic, and Cainozoic. The Deutozoic 

 opens with a great development of fishes ; the Mesozoic 

 with the expansion of reptiles ; and the .Cainozoic with 

 that of mammals and birds. The passage between the 

 Deutozoic and the Mesozoic is not marked by the rapid 

 extinction of any group, because the new development 

 of life was on land, and did not interfere with the 

 fishes of the ocean. But the great expansion of 

 mammals and birds was coincident with the exter- 

 mination of the huge Mesozoic reptiles of the land, 

 the sea, and the air ; so that the break between the 

 life of the Cretaceous and that of the Eocene is greater 

 than that between any other two consecutive periods. 



