Ch. X] of tertiaky formations. 113 



cases there may be scarcely a single sliell common to tlie tliree different 

 deposits, we do not hesitate to refer them all to one period (the Post- 

 Pliocene), because of the very close agreement of the fossil species in 

 every instance with those now living in the contiguous seas. 



To take another example, where the fossil fauna recedes a few steps 

 farther back from our own times. We may compare, first, the beds of 

 loam and clay bordering the Clyde in Scotland (called glacial by some 

 geologists), secondly, others of fluvio-marine origin near Norwich, and, 

 lastly, a third set often rising to considerable heights in Sicily, and we 

 discover that in every case more than three-fourths of the shells agree 

 with species still living, while the remainder are extinct. Hence we may 

 conclude that all these, greatly diversified as are their organic remains, 

 belong to one and the same era, or to a period immediately antecedent 

 to the Post-Pliocene, because there has been time in each of the areas 

 alluded to for an equal or nearly equal amount of change in the marine 

 testaceous fauna. Contemporaneousness of origin is inferred in these 

 cases, in spite of the most marked differences of mineral character or 

 organic contents, from a similar degree of divergence in the shells from 

 those now living in the adjoining seas. The advantage of such a test 

 consists in supplying us with a common point of departure in all coun- 

 tries, however remote. 



But the farther we recede from the present times, and the smaller the 

 relative number of recent as compared with extinct species in the ter- 

 tiary deposits, the less confidence can we place in the exact value of such 

 a test, especially when comjjaring the strata of very distant regions ; for 

 we cannot presume that the rate of former alterations in the animate 

 world, or the continual going out and coming in of species, has been 

 everywhere exactly equal in equal quantities of time. The form of the 

 land and sea, and the climate, may have changed more in one region 

 than in another ; and consequently there may have been a more rapid 

 destruction and renovation of species in one part of the globe than 

 elsewhere. Considerations of this kind should undoubtedly put us on 

 our guard against relying too imphcitly on the accuracy of this test ; 

 ye; it can never fail to throw great light on the chronological re- 

 lations of tertiary groups with each other, and with the Post-Pliocene 

 period. 



We may derive a conviction of this truth not only from a study of 

 geological monuments of all ages, but also by reflecting on the tendency 

 which prevails in the present state of nature to a uniform rate of simul- 

 taneous fluctuation in the flora and fauna of the whole globe. The 

 gTOunds of such a doctrine cannot be discussed here, and I have ex- 

 plained them at some length in the third Book of the Principles of 

 Geology, where the causes of the successive extinction of species are 

 considered. It will be there seen that each local change in climate and 

 physical geography is attended with the immediate increase of certain 

 species, and the limitation of the range of others. A revolution thus 

 effected is rarely, if ever, confined to a hmited space, or to one geograph- 



