GEOLOGICAL INFLUENCES. 667 



quiries, to discover the probable aboriginal localities of species of 

 plants. 



It is impossible to say in what part of the globe plants first appeared. 

 Probably they grew on lands now submerged beneath the ocean, or, still 

 more likely, they were perishable aquatic plants which have left no trace 

 of their existence. Geology teaches that the dry land of the globe has 

 been successively elevated and depressed below the surface of the sea at 

 various epochs since plants were first created j hence there have existed 

 successive and variable centres of creation. There may have existed 

 means of communication between different centres, so that species may 

 have passed over from one to another, and in this way survived in a new 

 locality the destruction of their birthplace. 



Species have made their appearance successively during different 

 geological epochs, and have had more or less extended duration. 



Probably most of our existing species date from an epoch 

 anterior to that at which the existing continents acquired their 

 present configuration. A large proportion of the present genera 

 were in existence before the end of the Secondary period. 



They may have spread widely in ancient times, and their area 

 may have been broken up subsequently by obstacles now insur- 

 mountable. They may have been transported in past ages by 

 causes not now in operation. 



Thus the disjunction of certain alpine and arctic species, that of aquatic 

 or marsh-plants in different countries, that of large-seeded plants in islands 

 and more or less distant continents, may be explained by their antiquity 

 or former wide diffusion, as well as by supposing creation at various 

 points. 



The species at present confined within small areas, in spite of 

 means of transport or continuity of land and suitable climate, 

 would appear to be those of most recent creation ; that is, they 

 seem to have originated since the existing continents were formed. 

 Widely spread species, on the other hand, which are difficult of 

 transport are probably the most ancient. 



In the comparison of successive geological formations, it 

 appears that the earliest plants were chiefly species of simple 

 organization and few in number and that by degrees more highly 

 organized plants were added and replaced many of the earlier ones, 

 which perished. In existing vegetation the simpler kinds seem 

 to be the most ancient, and those of more complex structure 

 more recent, judging from the wider diffusion of the former than 

 the latter. 



Ligneous plants established themselves in northern and temperate 

 countries at an epoch when the climate must have been more humid and 

 more cloudy than at present. At the present time, regions in the South 

 of Europe, North Africa, the Canaries, the Southern United States, and 



