BOOK VII 



THE INFLUENCE OF MIGRATIONS 

 CHAPTER XXVI 



THE MIGRATIONS OF MARINE ANIMALS 



The relation of the migration of beings to Palseogeography The migra- 

 tions of marine animals The influence of ocean currents The 

 displacement of foreshores and the migration of the environment 

 The part played by incursions of the sea. 



WHEN palaeontologists attempt to trace, through 

 earlier and earlier geological periods, the series of 

 animal forms which represent the natural evolution 

 of a branch, they are nearly always stopped, after 

 a more or less long geological course, by an 

 absolutely impassable hiatus. Just as we have seen 

 branches at their highest point end by abrupt 

 extinction, so it seems that the majority of them 

 appear abruptly and complete as if they had been 

 created altogether in the region under observation. 

 This apparent arrest at the outset of the evolution of 

 each branch is explained by the sudden arrival of 

 the group under notice in the region of the globe 

 under study. It is expedient to state precisely 

 this general law of the changes of faunas through 

 migrations, and to show its great importance. 

 The importance of the migrations of terrestrial 

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