726 TEXTBOOK OF BOTANY 



tinent was land above sea level than at present. During the early 

 Paleozoic w^hen there was widespread submergence of the continent 

 only a small part of the present continent was above water. The Pale- 

 ozoic was the period during which the thick and extensive beds of 

 limestone in the eastern states were deposited. 



Obviously, all these changes — the elevation of the continent, the up- 

 lift of mountains, the periods of great volcanic activity and long interven- 

 ing periods of erosion and deposition, and the accompanying changes 

 in climate — were favorable to the growth and survival of some kinds 

 of plants but led to the death of others. These same phenomena oc- 

 curred on other continents, and they are occurring at the present per- 

 haps as rapidlv on the average as they did in the past. The processes 

 by which plants change and the manner in which environment may 

 affect their survival and rate of change are discussed in Chapters XXXIX 

 and XL, 



At some time North America was broadly connected with Asia 

 through the Alaskan peninsula. Likewise, land connected North and 

 South America during certain millions of years, and during certain 

 other millions of years the ocean covered parts of Central America. 

 Land connections for long periods of time make possible the migration 

 of species from one continent to another. The severing of these con- 

 nections leads to the isolation of the plants of one continent from those 

 of another. Isolation bv restricting hybridization makes possible the 

 independent evolution of species, genera, and families, and the conse- 

 quent diversity of continental floras. 



These facts help to account for the peculiarly distinct floras of Aus- 

 tralia and New Zealand, which have been isolated from the other con- 

 tinents since the first appearance of the mammals. Eastern China and 

 Japan have forests of the same genera of trees as those of the eastern 

 United States. They are both remnants of a forest that was once con- 

 tinuous through Alaska and Siberia. 



Facts gleaned from the fossil record prove beyond a doubt how long 

 plants have lived on the earth. Studies of fossils have definitely proved 

 that the vegetation of the present is derived from that of the past. The 

 present flora is much more like that of the Pliocene than like that of 

 the upper Cretaceous, and it has still less in common with that of the 

 lower Cretaceous horizons. New kinds of plants evolve and older ones 

 become extinct. 



