PKIM^VAL CLIMATE OF THE EAETH. 36 



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The former were separated by geological depressions 

 the latter by elevations. Now, if we further consider how 

 often and how unequally these alternating elevations and 

 depressions occur on the different parts of the earth, and how, 

 in consequence of this, the boundaries of the geographical 

 tracts of distribution of species become changed, and if 

 we further consider in what exceedingly various ways the 

 active and passive migrations of organisms must have been 

 influenced by them, then we shall be in a position to com- 

 pletely understand the great variety of the picture which 

 is at present offered to us by the distribution of animal 

 and vegetable species. 



There is yet another important circumstance to be men- 

 tioned here, which is likewise of great importance for a 

 complete explanation of this varied geographical picture, 

 and which throws light upon many very obscure facts, 

 which, without its help, we should not be able to compre- 

 hend. I mean the gradual change of climate which has 

 taken place during the long course of the organic history of 

 the earth. As we saw in our last chapter, at the beginning 

 of organic life on the earth a much higher and more equal 

 temperature must have generally prevailed than at present. 

 The differences of zones, which in our time are so very 

 striking, did not exist at all in those times. It is probable 

 that for many millions of years but one climate prevailed 

 over the whole earth, which very closely resembled, or even 

 surpassed, the hottest tropical climate of the present day. 

 The highest north which man has yet reached was then 

 covered with palms and other tropical plants, the fossil re- 

 mains of which are still found there. The temperature of 

 tliis climate at a later period gradually decreased ; but still 



