324 Professor Edward Forbes on the Light thrown on 



shallow sea, and the fish remains often found in such rocks, 

 are explained, in a great measure, by these facts. 



V. Beds of marine animals do not increase to an indefinite 

 extent. Each species is adapted to live on certain sorts of sea- 

 bottmn only. It may die out in consequence of its own increase 

 changing the ground. — Thus, a bed of scallops, Pecten opercu- 

 laris, for example, or of oysters having increased to such an 

 extent that the ground is completely changed, in consequence 

 of the accumulation of the remains of dead scallops or oysters, 

 becomes unfitted for the further sustenance of the tribe. The 

 young cease to be developed there, and the race dies out, and 

 becomes silted up or imbedded in sediment, when the ground 

 being renewed, it may be succeeded either by a fresh colony of 

 scallops, or by some other species or assemblage of species. This 

 " rotation of crops," as it were, is continually going on in the 

 bed of the sea, and aifords a very simple explanation of the 

 alternation of fossiliferous and nonfossiliferous strata ; organic 

 remains in rocks being very rarely scattered through their 

 substance, but arranged in layers of various thickness, inter- 

 stratified with layers containing few or no fossils. Such in- 

 terstratification may, in certain cases, be caused in another 

 way, to-wit, by the elevation or subsidence of the sea- bottom, 

 and the consequent destruction of the inhabitants of one re- 

 gion of depth, and the substitution of those of another. It is 

 by such efi^ects of oscillation of level, we may account for the 

 repetition, at intervals, in certain formations of strata indi- 

 cating the same region of depth. 



VI. Animals having the greatest ranges in depth have 

 usually a great geographical., or else a great geological range, or 

 both. — I found that such of the Mediterranean testacea as 

 occur both in the existing sea, and in the neighbouring ter- 

 tiaries, were such as had the power of living in several of the 

 zones in depth, or else had a wide geographical distribution, 

 frequently both. The same holds true of the testacea in the 

 tertiary strata of Great Britain. The cause is obvious : such 

 species as had the widest horizontal and vertical ranges in 

 space, are exactly such as would live longest in time, since they 



