CHAPTER IV. 



Distribution of Organisms in Geologic Time, with 

 Reference to Their Physical Environment. 



§ 1. Definition of Facies. 



We have now to consider a new category of problems resulting 

 from palaeontologic studies, problems which pertain more espe- 

 cially to the relations of that science with stratigraphic Geology ; 

 they arise from the comparison of fossils of different groups 

 which have existed at the same epoch in different localities. It 

 has long been admitted as a fundamental principle of Strati- 

 graphy that, in a general sense, deposits of the same age present 

 the same fossils. But it is evident that at the present epoch such 

 a principle could not be applied ; the faunas and floras are local- 

 ized in more or less extended regions, and there are only a few 

 forms whose distribution is universal. It has been thus at all 

 epochs; never have the fauna and flora of the globe been 

 identical over its whole surface. 



Biologic provinces are defined for any given epoch by the con- 

 dition that at least one-half of the fauna and flora be distinct. 



The term facies is applied to the entirety of the lithologic and 

 palaeontologic characteristics resulting from the external condi- 

 tions which determine the existence of any particular fauna or flora 

 for a given region. These characters are defined by physical con_ 

 ditions, such as climate, altitude or bathymetric. depth, the geo- 

 logical or chemical nature of the medium. We may add, also, 

 indirect factors relating to the preceding. By virtue of the laws 

 of the struggle for existence, the variations of every living being 

 depend in a certain measure on the nature of the entire assem- 

 blage of the beings which live with it; the general characteristics 

 of the fauna or flora of a region therefore influence the conditions 

 of existence of each of the organisms which compose it, so that 

 the differences which, in consequence of the influence of the 



