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. 
