18(5 FLUCTUATIONS IN FAUNA AND FLORA. [Ch. XIII. 



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 comparing 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 implicitly 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 

 grounds 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 limited space, or to one geograph- 

 ical province of animals or plants, but affects several other surrounding 

 and contiguous provinces. In each of these, moreover, analogous alter- 

 ations of the stations and habitations of species are simultaneously in 

 progress, reacting in the manner already alluded to on the first province 

 Hence, long before the geography of any particular district can be essen- 

 tially altered, the flora and fauna throughout the world will have been 

 materially modified by countless disturbances in the mutual relation of 

 the various members of the organic creation to each other. To assume 

 that in one large area inhabited exclusively by a single assemblage of 

 species any important revolution in physical geography can be brought 

 about, while other areas remain stationary in regard to the position of 

 land and sea, the height of mountains, and so forth, is a most improba- 

 ble hypothesis, wholly opposed, to what we know of the laws now 

 governing the aqueous and igneous causes. On the other hand, even 

 were this conceivable, the communication of heat and cold between dif- 

 ferent parts of the atmosphere and ocean is so free and rapid, that the 



