90 Dr 6006*8 Geological Ohservatiotis. 



the cause of the differences observed, not only between the pre- 

 sent vegetable and animal creations, and those similar creations 

 which are buried in the earth, but also of those known varia- 

 tions in these last creations ? Geology has shewn, that the far- 

 ther we penetrate into the crust of the earth, the more simplicity 

 do we observe in the vegetable and animal productions, and the 

 more uniformity must the surface of the earth formerly have 

 presented in its creations. This can only be a consequence of 

 the greater equality of temperature which took place formerly 

 over the whole terrestrial surface, for the assigned causes had 

 then converted into warm zones those which are now cold or 

 temperate ; and they perhaps fgave the torrid zone a tempera- 

 ture much higher than it now possesses, while, at the same time, 

 certain parts of this zone could only have the temperatvire of the 

 other zones by various circumstances not understood. As the 

 jzones and climates gradually became established, the vegetables 

 and animals became diversified ; the vegetables of certain parts 

 of the earth also presently became extinct, because they no 

 longer found the climate which they required ; and, according 

 to their distribution at that period, and to the temperature ne- 

 cessary for their existence, some are no longer found upon the 

 -earth, others have degenerated, and some genera, or even per- 

 haps some species, requiring a pretty high temperature, still live 

 within the tropics. The animals which were not possessed of the 

 faculty of locomotion, must have experienced the same fate as 

 plants; but those species which could move, must have em- 

 ployed the means thus afforded them to travel, when it became 

 necessary, into climates which were favourable to their existence. 

 Some, suddenly brought into cold climates, in consequence of 

 volcanic elevations, or the retreat of the sea, must have perished 

 entirely, or must have remained in life only near the equator ; 

 others would soon have nowhere found the climate adapted for 

 them, and their races would have become extinct ; while, at the 

 same time, others would have descended, first, from the hills 

 into the valleys and flat country, and, afterwards, they would 

 no longer have been able to emigrate, when the temperature 

 would already have been too cold for them in these last locali- 

 ties. 



