RARE AND DOMINANT SPECIES 293 



dentally accumulated sample of a general fauna instead of the successive 

 appearance of a special fauna. 



The presence of an occasional associated species belonging normally 

 to the fauna of the formation in which the recurrent zone appears is not 

 antagonistic to the hypothesis, because the hypothesis proposes an invad- 

 ing of the territory already occupied by a normal fauna; and in case the 

 currents or other causes which brought about the shifting of the fauna 

 were not so completely different as to annihilate all evidence of the fauna 

 previously occupying the ground, some few species might be supposed 

 to hold over. Hence it is only necessary to find an abrupt change of the 

 majority of species to make the induction that the faunas have shifted 

 their habitats. 



Magna Faunas and local special Faunas 



The theory involves the further conception of grand general faunas 

 or magna-faunas which have their center of habitat and distribution in 

 permanent oceanic basins, as distinguished from the local, special and 

 (in geological strata) temporarily expressed faunas, such as we are ac- 

 customed to associate with individual geologic formations. 



In the case before us two such magna 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 ap- 

 pears dominantly in South America. 



In a case of recurrence in which there has been continuous sedimenta- 

 tion it is practically impossible to distribute all the species according to 

 their source of origin. I have found it possible, however, to distinguish 

 a few species as undoubtedly derived from a source different from that 

 of the prevailing fauna characterizing the beds both below and above the 

 recurrent zones. 



Fixed and fluctuating Characters 



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 are not only those characters by which one species 

 is distinguished from another, but they include others of generic, ordinal, 

 and even class rank, which may be of immense age in the race and mark 

 no special narrow stage of its history. 



