290 APPENDIX. 



Something may be said in favour of all these alleged causes ; 

 but as efficient in any important degree in producing the cold and 

 warm climates of the Tertiary period, the greater number of them 

 may be dismissed as incapable of effecting such results, or as 

 altogether uncertain with reference to the fact of their own occur- 



1. 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 influence 

 could have produced no appreciable effect, even in the 

 times of the earliest animals and plants, and certainly not 

 in the case of Tertiary floras and faunas. 



2. The obliquity of the ecliptic is not believed by astronomers 



to have changed to any great degree, and its effect would 

 be merely a somewhat different distribution of heat in 

 different periods of the year. 



3. Independently of astronomical objections, there is good 



geological evidence 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 pei"manence of the poles. 

 In like manner all the great folding's 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 followed the line of the existing Arctic currents from 

 the north-east to the south-west, throwing itself, for 

 example, along the line of the Appalachian uplifts in 

 Eastern America, and against the ridge of the Cordilleras 

 in the west. 



4. The effects of change of eccentricity and precession have 



been so ably urged by Croll, and recently by Ball, and 

 have so strongly influenced the minds of those who are 

 not working geologists, that they deserve a more detailed 

 notice. 



5. The heat of the sun is known to be variable, and the eleven 



years' period of sun spots has recently attracted much 

 attention as producing appreciable effects on the seasons. 

 There may possibly be longer cycles of solar energy ; or 

 the sun may be liable, like some variable stars, to 

 paroxysms of increased energy. Such changes are 

 possible, but we have no evidence of their occurrence, 

 and they could not account for periods of refrigeration of 

 limited duration like the Glacial age. 



6. It has been supposed that the earth may have at different 



