CLIMATE SINCE THE LATE CRETACEOUS 73 



isolated islands of such forest types. A few small fossil floras show 

 that the climate of southern California was cooler and moister than 

 at present. Such examples could be increased almost indefinitely 

 (see, for example, Murray, 1957). On the other hand, studies of fossil 

 pollen give evidence of the existence of oak-hickory forests during 

 Interglacial periods in areas around the Great Lakes that were 

 occupied by ice during the Glacial stages. 



The succession of Glacial and Interglacial stages was no doubt 

 the impetus for plant and animal dispersals north-south and up and 

 down in the mountainous areas. The distributional, ecological, and 

 speciational changes may be inferred, as follows. As the climate 

 ameliorated in an Interglacial interval, southern forms expanded 

 their habitat areas to the north and upward, while withdrawing 

 at the south. This northward extension continued until the climatic 

 trend was reversed. Isolated, relict areas of occupancy were left 

 scattered in favorable, or at least tolerated, locations south (and 

 possibly west) of the main occupied area. These relict areas, for 

 species dispersing northward, were on the higher elevations and on 

 cool north slopes, and may have ofTered especially favorable condi- 

 tions for the beginning of further speciation. Uninhabited areas, 

 newly exposed by the retreating ice, may have presented somewhat 

 different environments and changing competition to the vanguard 

 of the northward dispersal. With a turn toward advance of the ice 

 sheets, areas of occupancy tended to be driven southward, posing 

 new competitors and, perhaps, putting a premium on adaptability 

 to unaccustomed foods. There were many changes in local physio- 

 graphic barriers, ice sheets, rivers, and lakes. 



Pollen studies indicate that vegetational changes in middle lati- 

 tudes were marked. There is evidence that the prairies moved 

 northward and, in response to warmer and drier conditions, also 

 eastward during Interglacials. During pluvial Glacial stages prairies 

 invaded the eastern areas of our south-western deserts. During such 

 times of cyclic climatic changes the stimuli to admixture, hybridi- 

 zation, and natural selection must have been Intensified. The effects 

 on life were naturally greatest along the southern extension of the 

 glacial fronts, but these effects are found far to the south (in the 

 mountains of Mexico, for instance). Apparently the most favored 

 areas climatically were along the Pacific Coast and the southern 

 Atlantic seaboard. It is probable that there were dispersals back and 



