668 GEOLOGY, 



conditions which are congenial to them, and thus furnish the basis for 

 interpreting such conditions in the past, so far as the floras are well 

 preserved. The faunas, especially the land faunas, being primaril}. 

 dependent on the floras, furnish a basis for interpretations of like import. 



B. The Influence of Geographic Conditions on the Evolution of 



Floras and Faunas. 



The geographic features of the earth impose on organisms a com- 

 plex series of influences which modify the evolution of life and produce 

 faunal and floral variation on a large scale. The larger assemblages of 

 hfe, which inhabit a continent or dwell in a great sea, are designated 

 faunas and floras, as well as the smaller assemblages just discussed, but 

 obviously in a broader and in a different sense. The disseverance 

 of the land by the sea, or of the sea by the land, isolates the hfe and 

 forces independent development. The introduction of cold zones, 

 desert tracts, or other potent climatic belts has somewhat the same effect. 

 So, measurably, does the raising of a mountain range or a plateau, or 

 the sinking of critical portions of the sea-bottom. 



The development of provincial and cosmopolitan faunas/ — If a 

 region is isolated from other regions by the cutting off of all ready means 

 of intermigration, as by the formation of an island from what had been 

 a peninsula, or of an inland sea from what had been a bay, the flora and 

 fauna are developed by themselves without much influx of other forms, 

 and hence become local or provincial. This is usually more marked 

 in the case of the fauna than of the flora, because the latter has more 

 ample means of dispersion, on the whole, and so the fauna may for 

 convenience be taken as the type. A good illustration is the native 

 fauna of Australia which was once connected with Asia, but has long 

 been separated from it. Previous to importations by man, this conti- 

 nent had a very peculiar and distinct fauna, descended from its Mesozoic 

 inhabitants. Most of the isolated islands have peculiar faunas, but in 

 many cases they were isolated from the beginning, having been built 

 up by volcanic action from the bottom of the sea, and their faunas are 

 due to the accidents of transportation and to the development of these 

 sporadic forms in isolation.^ 



^ Chamberlin. A Systematic Source of Evolution of Provincial Faunas, Jour, of 

 Geol., Vol. YI, 1898, pp. 597-609. 

 * Wallace. Island Life. 



