RELATIONS AMONG SPECIES 



381 



Central America. However, most of these mammals 

 likely never moved north of the Tropics. Southern 

 taxa that may have invaded the United States include 

 opossums among the marsupials; the capybara and 

 porcupine among the primitive rodents; and arma- 

 dillos, glyptodonts, and ground sloths among the 

 edentates. 



The opening of the Panamanian land bridge al- 

 lowed rapid and extensive invasion of South America 

 bv North American species. Consequently, many of 

 the primitive types of South America became extinct 

 and were replaced by northern taxa. 



Pleistocene. The Pleistocene, a brief epoc of about 

 one million years, was the time of great mixing of 

 floras and faunas. The drasic oscillations of life 

 brought about by the southward advances (Figure 

 19.18) and northward retreats (Figure 19.19) of gla- 

 ciers caused widespread extinction and intermin- 

 gling of species. For example, advances produced 

 progressive local changes in floras from one extreme 



Figure 19.18 Pleistocene paleogeogrophy and geofloras ot the time 

 of maximum glociotion. Symbols as in Figure 19.16. White areas of 

 continent indicote ice-covered areas and black areas indicate lakes. 



Figure 19.19 Pleistocene paleogeogrophy and geofloras at a time of 

 maximum warmth showing Greenland ice. Symbols as in Figure 19.16. 



to another (cliseral shifts). At one time Florida had 

 a Boreal Forest. 



When glaciers retreated, southern species traveled 

 northward. However, relicts often were left in cer- 

 tain southern and western localities. In addition, 

 newly ice-freed areas often presented entirely new en- 

 vironments, characterized by restricted invasion, 

 much evolution, and, later, new floral and faunal ele- 

 ments. Finally, the Grassland moved north and east, 

 a movement that caused much mixing with other 

 floras, especially the Deciduous Forest. Under such 

 conditions there could be nothing but the remarkable 

 instances of change in composition and evolution that 

 are documented. 



Southward movement of glaciers also had its ef- 

 fects. Some species were able to survive upon south- 

 ern slopes. In many species, both those remaining 

 and those moving, survival required the fortunate ac- 

 quisition of hereditary changes. Some did evolve and 

 persist in this manner; other did not and became 

 extinct. Also during these glacier advances, the 



