CHAPTER 2 



EVOLUTION AND THE NORTH 

 ATLANTIC SEA-BIRDS 



GEOLOGISTS DIFFER in their opinions of the origin of the Atlantic 

 Ocean. The followers of the geomorphologist Alfred ^Vegener 

 believe that it is a real crack in the earth's crust whose lips have drifted 

 away from each other, and this opinion is lent verisimilitude by the 

 neat way in which the east coast of the Americas can be applied to, 

 and will fit with extraordinary exactitude, the west coast of Europe 

 and Africa. It must be stated that, while the present opinion of most 

 geographers is that the resemblance of the Atlantic to a drifted crack 

 is purely coincidental, this is not shared by all students of animal 

 distribution and evolution, some of whom, find the Wegener theor)^ the 

 most economical hypothesis to account for the present situation. 



Whatever the truth is, there is no doubt that the boundaries of the 

 Atlantic, and their interconnections, have varied considerably; thus half- 

 way through the Cretaceous Period, about ninety million years ago (dur- 

 ing this long period nearly all the principal orders of birds evolved), 

 there were bridges between Europe, Greenland and Eastern North 

 America cutting the Arctic Ocean froml the North Atlantic 

 completely; and from then until the late Pliocene — perhaps only 

 two million years ago — there was no continuous Central American 

 land bridge, but a series of islands. 



Our present knowledge of the tree of bird evolution owes much 

 to Alexander Wetmore and his school, who have so notably added 

 to our knowledge of fossil birds during the last twenty years, especially 

 in North America. Birds do not appear very frequently in the sedi- 

 mentary rocks — their fossil population does not generally reflect 

 their true population in the same way as that of mammals is reflected. 

 However, if land-birds are rare in the beds, water-birds are relatively 



