PROGRESS OF TEREESTRIAL LIFE 7 



of land and sea, have entailed various modifications of cli- 

 mate beyond those dependent on latitude ; while a yet fur- 

 ther series of such modifications have been produced by 

 increasing differences of elevation in the land, which have 

 in sundry places brought arctic, temperate, and tropical 

 climates to within a few miles of each other. And the 

 general result of these changes is, that not only has every 

 extensive region its own meteorologic conditions, but that 

 every locality in each region differs more or less from oth- 

 ers in those conditions, as in its structure, its contour, its 

 soil. Thus, between our existing Earth, the i^henomena of 

 whose varied crust neither geographers, geologists, miner- 

 alogists, nor meteorologists have yet enumerated, and the 

 molten globe out of which it was evolved, the contrast in 

 heterogeneity is sufficiently striking. 



When from the Earth itself we larn to the plants and 

 animals that have lived, or still live, upon its surface, we 

 find ourselves in some difficulty from lack of facts. Tliat 

 every existing organism has been developed out of the 

 simple into the complex, is indeed the first established 

 truth of all ; and that every organism that has existed was 

 similarly developed, is an inference which no physiologist 

 will hesitate to draw. But when we pass from individual 

 forms of life to Life in general, and inquire whether the 

 same law is seen in the ensemble of its manifestations, — 

 wnether modern plants and animals are of more hetero- 

 geneous structure than ancient ones, and whether the 

 Earth's present Flora and Fauna are more heterogeneous 

 than the Flora and Fauna of the past, — we find the evi- 

 dence so fragmentary, that every conclusion is open to 

 dispute. Two-thirds of the Earth's surface being covered 

 by water ; a great part of the exposed land being inaccess- 

 ible to, or untravelled by, the geologist ; the greater part 

 of the remainder having been scarcely more than glanced 

 at ; and even the most famiUar portions, as England, hav- 



