MATTJJIJW. CLIMATE AND EVOLUTION 309 



prevented the populating of Cuba from North America, while facilitating 

 invasions from South and Central America. The present set of currents 

 reduces the probability of mammals reaching Madagascar from the Afri- 

 can mainland, while increasing the chances of Oriental animals reaching 

 it. It reduces materially the opportunities for Australian fauna to reach 

 New Zealand. 



We have no adequate data on which to base theories as to the former 

 set of oceanic currents. A worldwide uniformity of climate would prob- 

 ably reduce the north and south movement of the waters; the east and 

 west element of their motions is conditioned by the rotation of the earth, 

 and its velocity would be reduced proportionately to the north and south 

 movements; so that a more uniform climate would bring about a reduc- 

 tion of velocity rather than change in direction. The third principal 

 conditioning element is the conformation of the continents, and doubtless 

 the flooding of great areas and the opening up of broad though shallow 

 passageways between seas now separated would profoundly modify the 

 surface currents in many regions. The opening of a broad passage be- 

 tween North and South America would allow the Caribbean current to 

 pass into the Pacific instead of being deflected northward and eastward 

 along the shores of the Gulf of Mexico to find an outlet between Cuba 

 and Florida. The absence of this initial part of the Gulf Stream would 

 obviously be unfavorable to North or Central American animals reaching 

 western Cuba. The great equatorial current would sweep across from 

 Africa along the northern coast of South America, and uninterruptedly 

 into the Pacific; transportation from Africa to South America or from 

 South or Central America to the Galapagos Islands would thus be facili- 

 tated. 



Dispersal of ]\Iammalia 



MANKIND 



We may with advantage begin our review of the special evidence in 

 support of our theory with the migration history of man. This is the 

 most recent great migration ; it has profoundly affected zoogeographic 

 conditions ; it is the one where our data are most complete and accurate ; 

 we can perceive its causes and conditions most clearly, and we have a 

 great deal of corroborative evidence in history and tradition. 



All authorities are to-day agreed in placing the center of dispersal of 

 the human race in Asia. Its more exact location may be differently in- 

 terpreted, but the consensus of modern opinion would place it probably 

 in or al)out tlie great plateau of central Asia. In this region, now barren 



