A good deal of misconception has arisen concerning the geographical distribution of former faunas, owing to the tendency to compare them exclusively with the littoral faunas of the present day. These littoral faunas have a comparatively limited geographical distribution, the forms of one marine province often differing considerably from those of an adjoining one, and still more widely from one which is remote, so that anyone confronted with the relics of faunas from the existing Australian and European seas, would find no indications furnished by identity of species that the faunas were contemporaneous. Recent researches have shown, however, that the creatures whose remains are deposited at some distance from the coast-line have a much stronger resemblance to one another than the littoral organisms have, if the fauna of two distant areas be compared. It is still a moot point which will be discussed in a later chapter, how far the deep-sea deposits of modern times are represented amongst the strata of the geological column by deposits of similar origin. But it is certain that many of the ancient strata are not littoral deposits, and it will be found that it is by comparison of the faunas of the deeper-water deposits that the geologist correlates the strata of remote regions: where shallow water deposits are formed, the faunas differ markedly in different regions, and these shallow-water forms can only be correlated owing to their occurrence between deeper-water strata. Thus if strata _A_, _B_ and _C_ be found in one area, and the fauna of _A_ and _C_ are deep-water forms, those of _B_ being shallow-water forms, and in another area beds _A´_ contain the same fauna as _A_, and _C´_ the same fauna as _C_ whilst the fauna of _B´_ is different from that of _B_, we can nevertheless correlate the strata _B_ and _B´_ (if they be conformable with the underlying and overlying beds), because of the identity of age of the associated beds in the two areas. It will possibly be found that the strata _A_ and _C_ can be further subdivided into _A_{1}_, _A_{2}_, ... &c. _C_{1}_, _C_{2}_, ... by the existence of minor faunas, which are comparable in the two cases, but such subdivisions may not be established in the case of the beds _B_ and _B´_.