98 TESTS OF THE DIFFERENT AGES [Ch. IX, 



age of a formation, or of the contemporaneous origin of two deposits in 

 distant places, under very much the same restrictions as the test of min- 

 eral composition. 



First, the same fossils may be traced over wide regions, if we examine 

 strata in the direction of their planes, although by no means for indefi- 

 nite distances. 



Secondly, while the same fossils prevail in a particular set of strata 

 for hundreds of miles in a horizontal direction, we seldom meet with the 

 same remains for many fathoms, and very rarely for several hundred 

 yards, in a vertical line, or a line transverse to the strata. This fact has 

 now been verified in almost all parts of the globe, and has led to a con- 

 viction, that at successive periods of the past, the same area of land and 

 water has been inhabited by species of animals and plants even more 

 distinct than those which now people the antipodes, or which now co- 

 exist in the arctic, temperate, and tropical zones. It appears, that from 

 the remotest periods there has been ever a coming in of new organic 

 forms, and an extinction of those which pre-existed on the earth ; some 

 species having endured for a longer, others for a shorter, time ; while 

 none have ever reappeared after once dying out. The law which has 

 governed the creation and extinction of species seems to be expressed in 

 the verse of the poet, — 



Natura il fece, e poi ruppe la stampa. Ariosto. 

 Nature made him, and then broke the die. 



And this circumstance it is which confers on fossils their highest value as 

 chronological tests, giving to each of them, in the eyes of the geologist, 

 that authority which belongs to contemporary medals in history. 



The same cannot be said of each peculiar variety of rock ; for some 

 of these, as red marl and red sandstone, for example, may occur at once 

 at the top, bottom, and middle of the entire sedimentary series ; exhib- 

 iting in each position so perfect an identity of mineral aspect as to be 

 undistinguishable. Such exact repetitions, however, of the same mix- 

 tures of sediment have not often been produced, at distant periods, in 

 precisely the same parts of the globe ; and even where this has hap- 

 pened, we are seldom in any danger of confounding together the monu- 

 ments of remote eras, when we have studied their imbedded fossils and 

 their relative position. 



It was remarked that the same species of organic remains cannot be 

 traced horizontally, or in the direction of the planes of stratification for 

 indefinite distances. This might have been expected from analogy ; for 

 when we inquire into the present distribution of living beings, we find 

 that the habitable surface of the sea and land may be divided into a 

 considerable number of distinct provinces, each peopled by a peculiar 

 assemblage of animals and plants. In the Principles of Geology, I have 



■ endeavored to point out the extent ancT probable origin of these separate 



■ divisions ; and it was shown that climate is only one of many causes on 



