DISTRIBUTION OF MOLLUSCA IN TIME. -ill 



The dlstribntio7i of species in the strata (or in time), is like tlieir distri- 

 bution in space. Each is most abundant in one horizon, and becomes gradu- 

 ally less frequent iu the beds above and below ; the locality of the newest rock- 

 in which it occurs being often far renioved from that of the oldest,* 



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

 and diffuse themselves, is sufhcieutly intelligible. That, after attaining a 

 ■certain chmax 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 certaiu, and does not appear to vary 

 beyond moderate limits. 



The deep-sea shells (such as Rhi/nchoitella, Terehratula, and Yoldia), 

 enjoy a longer range iu time, as well as in space, than the litoral species ; 

 whilst the land and fresh-water shells are most remarkable for specific loii- 

 gevity.f 



In each stratum there are some fossils which characterize small subdivi- 

 sions 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, whicli 

 fulhlled their fuuctious, and are found in deposits formed under similar con- 

 ditions. {Forbes.) 



The total number of species is greater in the newest formations than iu 

 those of older date; but the ratio of increase has not been ascertained. | 



Distrlbut'wn of Genera in Time, The doctrine of the Identification of 

 strata by fossils derives its chief value from the fact that the development and 

 distribution of Genera is as much subject to law as the distribution of species ; 

 and so far as wc know, follows a similar law. 



Groups of strata, like the Zoological provinces, may be of various magni- 

 tudes ; and whilst the smaller divisions are characterized by peculiar species. 



estimated to represent lt'0,000 years, have been accumulated since the era of many 

 existing shells. The same may be said of the elevation of Mont Blanc, the formation 

 of the Mediterranean Sea and other grand physical events. The great cities of anti- 

 quity—Rome, Corinth, and Egyptian Thebes, stand upon raised sea-beds, or aiiuvial 

 deposits, containing recent shells. 



* M. AgassJz and Prof. E, Forbes have represented, diagramatically, the distribvi- 

 tion of genera in time, by making the horizontal lines (such as in p. 41 j) 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 maximd in the present seas are tliUs elpressed 



+ Land and Fresh-water shells of existing species are found with the fossil bories 

 of the JIuslodon and Mcijalonyx, in N. America. {Lijell.) 



X The number in each formation depends on the extent to whicli it has been inves- 

 tigated, and on the opinions entertained as to the .strata referable to it. Pfof. Phillips 

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

 •• Guide to Geology." 



T 3 



