100 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



glaciers of the Scandinavian mountains and of the Alps extended far 

 into the plains, and glaciers were present even in the intermediate 

 ranges. The climate of the lowlands was wholly different from that of 

 the present. A large part of North America, at the same time, was 

 covered by a continuous continental glacier. The most recent period 

 of glaciation was divided into interglacial periods during which ani- 

 mals intolerant of cold invaded northern regions only to be driven out 

 by a new advance of the ice sheet. Traces of glaciations in much more 

 remote epochs may be observed in many regions, even in the tropics. 

 The presence of coral reefs in northern latitudes also indicates a warmer 

 climate of the contemporary seas, for reef corals are now restricted to 

 the tropics and require a minimum water temperature of 20°C. Marine 

 deposits of Iceland and the Arctic of relatively recent periods exhibit 

 a fauna now found only far to the south. 



Since barriers are important for the evolution of animal forms and 

 have a direct effect upon distribution, it is evident that the past rear- 

 rangements of land and sea and of other connections and barriers must 

 have been of profound influence in the dispersal of animal life on the 

 earth's surface. Satisfactory explanations of present faunal relations 

 may depend on the recognition of the presence of former connections 

 or barriers. Zoogeography must reckon with time as well as with space. 5 



On the basis of the evolutionary theory, we must assume that, acci- 

 dental dispersal aside, every natural group, whether species, genus, or 

 family, must inhabit a continuous area, or an area that has at least 

 been continuous or connected at some time during the history of the 

 group. Such continuity is primarily a continuity in time, for the various 

 connections which have influenced the dispersal of the group need not 

 have been contemporaneous. Forms with a common ancestry must 

 originate at common centers of dispersal. 



There are certain modifying corollaries to be added to this basic 

 assumption. Wallace states that "Two identical species have never 

 been developed independently in widely separated areas," which ap- 

 plies equally to groups of higher rank. In the sense that different 

 ancestral forms could not give rise by convergence to identical descend- 

 ants, this is unquestionably valid, for the number of separate characters 

 which would have to fall in line for such an end is unthinkable. Never- 

 theless, identical varieties may arise from the same stock in localities 

 which are widely separated if they arise as modifications due to the 

 influence of the same conditions. The widely distributed appendiculate, 

 Frittilaria borealis, appears in the same subspecies (recognized as the 

 forma typica) in both the north and south polar seas and has unques- 



