THEORY CMP MR. LVELL. 413 



of the earth, from the equator to the polar 

 regions.* 



In his recent excellent work on geology, Mr. 

 Lyell refers all the past revolutions of organic 

 life to changes of climate, and the latter to 

 geological causes alone ; such as the gradual 

 shifting of sea and land, brought about by the 

 agency of volcanos, earthquakes, hot springs, 

 currents and waves of the ocean, the corroding 

 action of rains, rivers, springs, and chemical 

 decomposition. But if we admit with Mr. Lyell, 

 that the greater part of the dry land now scat- 

 tered over the northern hemisphere, may have 

 been formerly confined within the tropics, and 

 the mean temperature of the whole earth thus 



* For example, nearly all the fossil plants hitherto discovered 

 in the older formations consisted of ferns, coniferae, Equiseta- 

 cese, and Cycadeae ; which in all, comprised only about five 

 hundred species, that presented very little diversity of form. It 

 is even asserted by the most intelligent botanists who have care- 

 fully examined the subject, that for a long period, during which 

 the climate of the middle latitudes seems to have exceeded the 

 present temperature of the tropics, cycadese alone formed about 

 one third of the entire Flora ; whereas they now constitute only 

 one two thousandth of the whole. But in the present diversified 

 condition of the earth, as regards temperature, it contains about 

 one hundred thousand species of plants, and above five hundred 

 thousand species of animals. It would therefore appear that 

 diversity in the generic and specific character of organized bodies, 

 depends chiefly on varieties of climate and season, which differ 

 in all the higher latitudes, and owing to the influence of local 

 causes, are scarcely ever exactly alike, even in the same latitudes, 

 as will be shewn in the first chapter of Book V. This subject 

 opens a vast field of inquiry to those physiologists who may be 

 disposed to investigate the origin of organic species. 



