2l6 THE GENESIS AND MIGRATIONS OF PLANTS 



for but a small part of the phenomena, which require to be ex- 

 plained by physical changes affecting the earth as a whole, or 

 at least the northern hemisphere. Many theoretical views 

 have been suggested on this subject, which will be found dis- 

 cussed elsewhere, and perhaps the most practical way to deal 

 with them here will be to refer to the actual conditions known 

 to have prevailed in connection with the introduction and 

 distribution of the principal floras which have succeeded each 

 other in geological history. 



If we can assume that all the carbon now sealed up in lime- 

 stones and in coal was originally floating in the atmosphere 

 as carbon dioxide, then we would have a cause which might 

 seriously have affected the earlier land floras — that, for instance, 

 which may have existed in the Eozoic age, and those well 

 known to us in the Palaeozoic. Such an excess of carbonic 

 acid would have required some difference of constitution in 

 the plants themselves ; it would have afforded them a super- 

 abundance of wood-forming nutriment, and it would have 

 acted as an obstacle to the radiation of heat from the earth, 

 almost equal to the glass roof of a greenhouse, thus constituting 

 a great corrective of changes of temperature. Under such cir- 

 cumstances we might expect a peculiar and exuberant vegeta- 

 tion in the earlier geological ages, though this would not apply 

 to the later in any appreciable degree. In addition to this 

 we know that the geographical arrangements of our continents 

 were suited to the production of a great uniformity of cHmate. 

 Taking the American continent as the simpler, we know that 

 in this period there existed in the interior plateau between the 

 rudimentary eastern and western mountains a great inland 

 sea, so sheltered from the north that its waters contained hun- 

 dreds of species of corals, growing with a luxuriance unsur- 

 passed in the modern tropics. On the shores and islands of 

 such a sea we do not wonder that there should have been tree 

 ferns and gigantic lycopods. In the succeeding Carboniferous. 



