CHAPTER VII 

 HISTORICAL ZOOGEOGRAPHY 



The distribution of life is not satisfactorily explainable by the 

 present arrangement of barriers and land connections. There are fre- 

 quent examples of areas formerly connected now entirely separated. 

 North America, for example, has much more faunal resemblance with 

 Eurasia than with South America. The fauna of southern Victoria 

 agrees closely with that of Tasmania, and differs markedly from that 

 of northern Victoria and New South Wales. Many such facts are ex- 

 plained by the changes in the earth's surface in the course of geologic 

 time. The stratified rocks which cover great areas of the existing con- 

 tinents show that there have been extensive invasions of these land 

 areas by the sea. Conversely, numerous land masses now separated 

 from the adjoining continents, notably Great Britain and many East 

 Indian islands, can be shown to have been joined to them by dry land 

 in the past. It is generally accepted that Great Britain was united with 

 the European continent until recent times, geologically speaking, on 

 the evidence of the presence of submarine continuations of the British 

 and Norwegian river valleys to a depth of 60 m., of the presence of ter- 

 restrial deposits at a depth of 200 m. off Bremen, and of the presence 

 of the remains of large land mammals, such as the mammoth and 

 rhinoceros, on the Dogger Banks in the North Sea, where they are 

 frequently brought up by fishermen. 1 The invasion of the continents 

 by relatively shallow inland seas need not affect the general perma- 

 nence of outline of the great continental blocks, whose limits are defined 

 by the continental shelves rather than by existing shore lines. 



The older concepts of the origin and evolution of life, based on the 

 nebular hypothesis, require complete revision in the light of the much 

 more adequate planetesimal hypothesis of Chamberlin and Moulton. 

 Chamberlin's further theories of the larger outlines of geological his- 

 tory are of special importance to historical zoogeography. According to 

 these theories, the earth has passed through a number of climatic 

 cycles. From the extreme of humidity, warmth, and uniformity, which 

 corresponds to base-level erosion of the continents, accompanied by 

 extensive transgressions of the continental borders by shallow inland 

 seas, the cycle passes to the opposite extreme of cold, aridity, and 



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