248 THE GEOLOGICAL HISTORY OF PLANTS. 



require to be explained by physical changes affecting the 

 earth as a whole, or at least the northern hemisphere. 

 Many theoretical views have been suggested on this sub- 

 ject, and perhaps the most practical way of disposing of 

 these will be first to set aside a number which are either 

 precluded by the known facts, incapable of producing 

 the effects, or altogether uncertain as to their possible 

 occurrence. 



1. In this class we may place the theory that the poles 

 of the earth have changed their position. Independently 

 of astronomical objections, there is good geological evi- 

 dence that the poles of the earth must have been nearly 

 in their present places from the dawn of life until now. 

 From the Laurentian upward, those organic limestones 

 which mark the areas where warm and shallow equatorial 

 water was spreading over submerged continents are so 

 disposed as to prove the permanence of the poles. In 

 like manner all the great foldings of the crust of the earth 

 have followed lines which are parts of great circles tangent 

 to the existing polar circles. So, also, from the Cambrian 

 age the great drift of sediment from the north has fol- 

 lowed the line of the existing Arctic currents from the 

 northeast to the southwest, throwing itself, for example, 

 along the line of the Appalachian uplifts in eastern 

 America, and against the ridge of the Cordilleras in the 

 west. 



2. Some of the above considerations, along with astro- 

 nomical evidence, prevent us from assuming any consid- 

 erable change in the obliquity of the axis of the earth 

 during geological time. 



3. That the earth and the sun have diminished in 

 heat during geological time seems probable ; but physical 

 and geological facts alike render it certain that this influ- 

 ence could have produced no appreciable effect, even in 

 the times of the earliest floras, and certainly not in the 

 case of Tertiary vegetation. 



