44 WIDE RANGE OF MARINE TESTACEA. [Ch. IV. 



contemporaneous origin of formations in distant countries. As 

 the same species of organic beings usually extend over wider 

 areas than deposits of a homogeneous composition, they are 

 more valuable in geological classification than mineral peculia- 

 rities ; but it fortunately happens, that where the one criterion 

 fails, we can often avail ourselves of the other. Thus, for 

 example, sedimentary strata are as likely to preserve the same 

 colour and composition in a part of the ocean reaching from the 

 borders of the tropics to the temperate zone, as in any other 

 quarter of the globe; but in such spaces the variation of species 

 is always most considerable. 



In regard to the habitations of species, the marine tribes are 

 of more importance than the terrestrial, not only because they 

 are liable to be fossilized in subaqueous deposits in the great- 

 est abundance, but because they have, for the most part, a 

 wider geographical range. Sometimes, however, it may hap- 

 pen, as we have shown, that the remains of species of some 

 one province of terrestrial plants and animals may be carried 

 down into two seas inhabited by distinct marine species ; and 

 here again we have an illustration of the principle, that when 

 one means of identification fails, another is often at hand to 

 assist us. 



In conclusion, we may observe, that in endeavouring to 

 prove the contemporaneous origin of strata in remote countries 

 by organic remains, we must form our conclusions from a great 

 number of species, since a single species may be enabled to 

 survive vicissitudes in the earth's surface, whereby thousands 

 of others are exterminated. When a change of climate takes 

 place, some may migrate and become denizens of other latitudes, 

 and so abound there, as to characterize strata of a subsequent 

 era. In the last volume we have stated our reasons for in- 

 ferring that such migrations are never sufficiently general to 

 interfere seriously with geological conclusions, provided we do 

 not found our theories on the occurrence of a small number of 

 fossil species. 



