114 FOSSIL SHELLS. [Ch. X 



ical province of animals or plants, but affects several other surrounding 

 and contiguous provinces. In each, of these, moreover, analogous alter- 

 ations of the stations and habitations of species are simultaneously in 

 progress, reacting in the manner already alluded to on the first province 

 Hence, long before the geography of any particular district can be essen- 

 tially altered, the flora and ftiuna throughout the world will have been 

 materially modified by countless disturbances in the mutual relation of 

 the various members of the organic creation to each other. To assume 

 that in one large area inhabited exclusively by a single assemblage of 

 species any important revolution in physical geography can be brought 

 about, while other areas remain stationary in regard to the position of 

 land and sea, the height of mountains, and so forth, is a most improba- 

 ble hypothesis, wholly opposed to what we know of the laws now 

 governing the aqueous and igneous causes. On the other hand, even 

 were this conceivable, the communication of heat and cold between dif- 

 ferent parts of the atmosphere and ocean is so free and rapid, that the 

 temperature of certain zones cannot be materially raised or lowered 

 without others being immediately afi'ected ; and the elevation or dimi- 

 nution in height of an important chain of mountains or the submergence 

 of a wide tract of land would modify the climate even of the antipodes. 



It will be observed that in the foregoing allusions to organic remains, 

 the testacea or the shell-bearing mollusca are selected as the most useful 

 and convenient class for the purposes of general classification. In the 

 first place, they are more universally distributed through strata of every 

 age than any other organic bodies. Those families of fossils which are 

 of rare and casual occurrence are absolutely of no avail in establishing 

 a chronological arrangement. If we have plants alone in one group of 

 strata and the bones of mammalia in another, we can draw no conclusion 

 respecting the afiinity or discordance of the organic beings of the two 

 epochs compared ; and the same may be said if we have plants and 

 vertebrated animals in one series and only shells in another. Although, 

 corals are more abundant, in a fossil state, than plants, reptiles, or fish, 

 they are still rare when contrasted with shells,' especially in the European 

 tertiary formations. The utility of the testacea is, moreover, enhanced 

 by the circumstance that some forms are proper to the sea, others to the 

 land, and others to freshwater. Rivers scarcely ever Ml to carry down 

 into their deltas some land shells, together with species which are at 

 once fluviatile and lacustrine. By this means we learn what terrestrial, 

 freshwater, and marine species coexisted at particular eras of the past ; 

 and having thus identified strata formed in seas with others which origi- 

 nated contemporaneously in inland lakes, we are then enabled to advance 

 a step farther, and show that certain quadrupeds or aquatic plants, found 

 fossil in lacustrine formations, inhabited the globe at the same period 

 when certain fish, reptiles, and zoophytes lived in the ocean. 



Among other characters of the molluscous animals, which render 

 them extremely valuable in settling chronological questions in geology, 

 may be mentioned, first, the wide geographical range of many S23ecies* 



