7 6 THE POPULAR SCIENCE MONTHLY 



fauna; not only have I found it necessary to establish identity of the 

 species in the recurrent zones with those of the initial zones, but it is 

 essential to show that the faunas as a whole are the same. 



To put this in another form of statement we must establish the fact 

 that not only the individual species have retained their specific char- 

 acters, but the further fact that the equilibrium of adjustment to each 

 other in the faunal community has not been changed, in order to prove 

 that the recurrent fauna is the direct successor of a fauna represented 

 in the rocks at a lower horizon. 



This has led to such distinction as rare and dominant species of the 

 fauna, and only as some such comparative frequency of the species in 

 the faunal combination is apparent can we be sure that we are not 

 considering an accidentally accumulated sample of a general fauna. 



The presence of occasional associated species belonging to the 

 normal fauna of the formation in which the recurrent zone appears is 

 not antagonistic to the theory, because the theory proposes an invading 

 of the territory occupied by the normal fauna, and whatever were the 

 causes which brought about the shifting of the fauna they were not 

 so completely different as to annihilate all evidence of the fauna previ- 

 ously occupying the ground. Hence it is only necessary to find an 

 abrupt change of the grand majority of species to make the induction 

 that the faunas have shifted their habitat. 



The theory involves the further conception of grand general faunas 

 which have their center of habitat and distribution in permanent 

 oceanic basins, as distinguished from the special and (in geological 

 strata) temporarily expressed faunas such as we are accustomed to as- 

 sociate with individual geologic formations. 



In the case before us two such general faunas are in evidence, one 

 of which in its dominant characteristics is traced westward into Iowa, 

 Idaho and Arizona and up the Mackenzie Eiver valley to the north and 

 across the polar regions to Eussia and northern Europe. The other 

 is traced eastward and southward into central and southern Europe 

 and also dominantly into South America. 



Although, with our present knowledge, it is not possible to deter- 

 mine in any temporary expression of marine faunas those particular 

 species which were derived from one from those derived from the other 

 grand source, it is possible to recognize numerous species which belong 

 to one center of distribution and others that belong normally to the 

 other. 



§10. Interpretation of tlieJ?acts. — It is also important to keep our 

 heads clear in interpreting the facts. 



It is only by close examination and comparison of the fossils them- 

 selves that identity of species or identity of faunas can be established. 



The fixed characters of species are not only the characters by which 



