PRIMAVAL CLIMATE OF THE EARTH. 363 
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 
this climate at a later period gradually decreased; but still 
