THE GENESIS AND MIGRATIONS OF PLANTS 225 



the southern hemisphere, yet these do not affect the general 

 facies. There are, for example, characteristic Lepidodendroids 

 in the Devonian and Carboniferous of Brazil, Australia, and 

 South Africa. If now we consider the plants a little more in 

 detail, coniferous and taxine trees grow now in very different 

 latitudes and climates. There is therefore nothing so very 

 remarkable in their occurrence. The great group of Cordaites 

 may have been equally hardy ; but it is noteworthy that their 

 geographical distribution is more limited. In Europe, for 

 example, they are more characteristic in France than in Great 

 Britain. Ferns and Lycopods and Mares'-tails are also cosmo- 

 politan, but the larger species belong to the warmer climates, 

 and nowhere at present do they become so woody and so com- 

 plex in structure as they were in the older geological periods. 

 At the present day, however, they love moisture rather than 

 aridity, and uniformity of temperature rather than extreme 

 light and heat. The natural inference would be that in these 

 older periods geographical and other conditions must have 

 conspired to produce a uniform and moist climate over a large 

 portion of the continents. The geographical conditions of 

 the Carboniferous age, and the distribution of animal life on 

 the sea and land, confirm the conclusion based on the flora. 

 Further, if, as seems probable, there was a larger proportion of 

 carbon dioxide in the atmosphere than at present, this would 

 not only directly affect the growth of plants, but would im- 

 pede radiation, and so prevent escape of heat by that means, 

 while the moisture exhaled from inland seas and lagoons and 

 vastly extended swamps, would tend in the same direction. 



It would, however, be a mistake to infer that there were not 

 local differences of climate. I have elsewhere 1 advocated the 

 theory that the great ridge of boulders, the New Glasgow con- 

 glomerate, which forms one margin of the coal field of Picton, 



* " Acadian Geology,'' Carboniferous ol Picton. 



