The Problems of Plant Distribution 237 



existed during the early geological ages, and prob- 

 ably were not very different from their living de- 

 scendants. 



Uniformity of the Early Floras. A notable fea- 

 ture of the primitive floras of the earth was their 

 uniformity. While at the present day different 

 regions possess very different floras, during the 

 Paleozoic era there seems to have been a practically 

 uniform flora throughout the greater part of the 

 earth. There is very little difference between the 

 Paleozoic fossils found in the arctic regions, and 

 those which occur in tropical beds, this being espe- 

 cially true of the Carboniferous fossils. 



Paleozoic Climate. Various explanations of the 

 apparently uniform climate that seems to have pre- 

 vailed have been made, one of the latest, and one 

 which has a good deal of plausibility, being that of 

 Manson, who believes that during the earlier geo- 

 logical epochs the earth was enveloped in a dense 

 layer of clouds which neutralized the effects of the 

 solar rays, the heat being mainly the result of direct 

 radiation from the earth itself, which would thus 

 have practically a uniform climate throughout its 

 whole extent. If this view is correct, it would have 

 to be assumed that the cloud envelope was suffi- 

 ciently transparent to admit enough of the light 

 rays for the existence of green plants. But it must 

 be remembered that the plants of this period, to 

 judge from the fossils, were mostly forms which 

 are able to grow with a limited amount of light, and 



