Turning now to the difference in organisms which results from difference of station, it will be sufficient to give a quotation from Woodward's _Manual of the Mollusca_ as an illustration:--"In Europe the characteristic genera of _rocky_ shores are _Littorina_, _Patella_, and _Purpura_; of sandy beaches, _Cardium_, _Tellina_, _Solen_; gravelly shores, _Mytilus_; and on muddy shores, _Lutraria_ and _Pullastra_. On rocky coasts are also found many species of _Haliotis_, _Siphonaria_, _Fissurella_, and _Trochus_; they occur at various levels, some only at the high-water line, others in a middle zone, or at the verge of low-water. _Cypræa_ and _Conus_ shelter under coral-blocks, and _Cerithium_, _Terebra_, _Natica_ and _Pyramidella_ bury in sand at low-water, but may be found by tracing the marks of their long burrows (Macgillivray)[37]." [Footnote 37: Woodward, S. P., _A Manual of the Mollusca_, p. 151.] The geologist will naturally select sporadic forms rather than endemic ones in comparing the strata of different areas, but how far differences in faunas are the result of existence at different times, and how far they are due to difference of conditions affecting contemporaneous organisms can only be discovered as the result of accurate observation. The main points to be regarded when comparing the successive faunas of different regions have been noticed in this and the preceding chapters, and it has been shown that as the evidence is cumulative, it requires the collection of a large number of facts obtained by observation of the strata before accurate inferences can be drawn.