268 INTRODUCTION TO EVOLUTION 



Centers of Dispersal 



There emerges from our discussion a picture of new forms arising from 

 old ones in certain regions and then migrating out from the "old home" in 

 search of new worlds to conquer. We must hasten to add that this migra- 

 tion and "search" are not to any considerable extent voluntary activities of 

 individual animals. Rather they represent slowly developing changes in- 

 volving many generations and produced by many factors, including even- 

 tual overcrowding of what we have spoken of as the "old home" but 

 what is more accurately termed the center of dispersal. 



As examples of dispersal from such a center we may cite the evolution- 

 ary history of placental mammals. We recall that the evolution and dis- 

 persal of placental mammals occupied the center of the stage during the 

 Cenozoic era (Chap. 10). 



If we look at the continents of Eurasia and North America on a map 

 drawn with the North Pole as a center (Fig. 12.4), we note that these lands 

 form the greater portion of a circle around the pole and if Bering Strait 

 were dry land would form one continuous land mass. Lowering the level 

 of the ocean, or raising its floor, by only 150 feet would provide a dry-land 

 connection across the strait. This land connection is believed to have arisen 

 at various times in the past, most recently during the Pleistocene period 

 when much water was locked up in continental glaciers. Most of Alaska 

 seems to have remained free of glaciers. Hence during various periods of 

 Pleistocene glaciation the land bridge could have afl'orded passage into the 

 New World to such creatures as bison, musk oxen, goats, moose, woolly 

 mammoths, and mastodons. Man himself probably also utilized this 

 bridge, which was last open for the period between 25,000 years and about 

 10,000 years ago (Hopkins, 1959). 



Most students of geographic distribution agree that this circumpolar land 

 mass has provided the route by which animals have been distributed to the 

 continents of the world. This land mass forms a hub from which three great 

 spokes radiate southward, to terminate, respectively, in South America, 

 Africa, and southeastern Asia with its adjoining islands (Fig. 12.4). What 

 was the point of origin and center of dispersal for modern orders of mam- 

 mals? Matthew (1939) concluded that these orders originated in the North 

 Temperate zone with its variable climate. Thus most of the evolution of the 

 horse occurred in North America, as did that of the camels (see below). 

 Other examples might be given. On the other hand, Darlington (1957) 

 marshalled evidence that animals, including mammals, are on the whole 

 more diverse and numerous in kinds in the tropics than they are in tem- 

 perate zones. He concluded that "the main center of dispersal of mam- 



