DISTRIBUTION OF MOLLTJSCA IN TrUE. 121 



and below ; the locality of the newest rock in which it occurs 

 being often far removed from that of the oldest. * 



That species should be created at a single spot, and gradually 

 multiply and diffuse themselves, is sufficiently intelligible. That, 

 after attaining a certain climax of development, they should 

 decline and disappear, is a fact involved in mystery. But even 

 if it depends on physical causes, and is not a law of all Being, its 

 operation is equally certain, and does not appear to vary beyond 

 moderate limits. 



The deep-sea shells (such as RliyncJionella, Terehratida, and 

 Yohlia) enjoy a longer range in time, as well as in space, than 

 the littoral species ; whilst the land and fresh-water shells are 

 most remarkable for specific longevity, f 



In each stratum there are some fossils which characterise 

 small subdivisions of rock, just as there are living species of 

 very limited range. 



"When species once die out they never reappear ; one evidence 

 of their having become extinct consisting in their replacement 

 by other species, which fulfilled their functions, and are found 

 in deposits formed under similar conditions. (Forbes.) 



The total number of species is greater in the newest forma- 

 tions than in those of older date ; but the ratio of increase has 

 not been ascertained. J 



Distribution of Genera iri Time. — The doctrine of the Identi- 

 fication of strata by fossils derives its chief value from the fact 

 that the development and distribution of genera is as much sub- 

 ject to law as the distribution of species ; and, so far as we know, 

 follows a similar law. 



Groups of strata, like the zoological provinces, may be of 

 various magnitudes ; and whilst the smaller divisions are cha- 

 racterised by peculiar species, the larger groups have distinc"^ 

 sub-genera, genera, and families, according to their size and 

 importance. 



William Smith himself observed that ' ' three principal families 

 of organised fossils occupy nearly three equal parts of Britain." 



* M. Agassiz and Professor E. Forbes have represented, diagrammatically, the 

 <!listribution of genera in time, by making the horizontal lines (such as in p. 124) swell 

 out in proportion to the development of the genera. Those whose commencement, 

 climax, and end are ascertained may be represented by a line of this kind — ^^^»— 

 Genera which attain their maxima in the present seas are thus expressed — ^^ 



t Land and fresh-water shells of existing species are found with the fossil bones of 

 the Mastodon and Megalonyx, in N. America. (Lyell.) 



X The number in each formation depends on the extent to wliich it has been investi- 

 gated, and on the opinions entertained as to the strata referable to it. Professor 

 Phillips has discussed this subject in his work on Devonian fossils (p. 165), and in tno 

 " Guide to Geology." 



a 



