40 THE LESSON OF EVOLUTION 



in the ocean, as well as on the land, became more 

 and more varied as time went on. 



This evolution may be compared to the growth of 

 a hypothetical tree , in which the shapes of the leaves 

 slowly alter year after year. The leaves which die 

 out represent the extinct genera and species ; while 

 the branches represent the classes and orders which 

 continue to exist. Occasionally a bough may die 

 also ; but it is not a regular occurrence as it is with 

 leaves. 



During the first half of the biological evolution, 

 when comparatively simple forms alone existed, the 

 struggle for existence was but slightly felt : species 

 existed for a long time, and spread far and wide over 

 the earth. But as the more complex and specialised 

 forms came into existence, during the second half of 

 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 

 between the faunas and floras of distant countries, 

 hardly recognisable during the Paleozoic 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 (Echinoidea), 

 the bivalve and univalve mollusca, the decapod 

 crustaceans, and the actinopterygian fishes ; with the 

 simultaneous decline of the Crinoidea and the 



