CONTEMPORANEOUS AND HOMOTAXIAL 1125 



bility of determining in any case the existence of contemporaneous 

 strata, and he coined the term lioinotaxis, signifying similarity of 

 order, to express the correspondence in succession rather than exact 

 time equivalence. Geikie also held that "strict contemporaneity 

 cannot be asserted of any strata merely on the ground of similarity 

 or identity of fossils" (14:608), and H. S. Williams, among 

 American authors, has most strenuously insisted on the impossibility 

 of recognizing strict contemporaneity among strata of widely sepa- 

 rated regions. Williams would refer formations not to a general 

 time scale, but to a stratigraphic scale, of which not "periods" but 

 systems are units. He advocates the revision of Rule 14 of the 

 U. S. Geological Survey quoted at the beginning of this chapter 

 so as to read (56:/j<?) : 



"The fundamental data of geologic history are: (i) the local 

 sequence of formations and (2) tJie similarity of the fossil faunas 

 of the formation of different provinces. Through correlation all 

 formations are referred to a standard strafi'graphic scale, of zvhich 

 the units are s^'stems." 



Contrary to the views of Huxley and other writers, some of 

 whom like Edward Forbes went so far as to assert that similarity 

 of organic content of distant formations is prima facie evidence, not 

 of their similarity, but of their difference of age, most modern 

 stratigraphers have come to believe in the possibility of essential 

 chronological equivalency of formations characterized by the same 

 faunas, recognizing at the same time the fact that such equivalency 

 is not necessarily indicated by the similarity of faunas, and that a 

 given fauna may appear earlier and continue longer in some sections 

 than in others. The rapidity of migration shown by modern faunas 

 indicates that, if the path is open and no barriers exist, widespread 

 migration or dispersal may occur within such short time limits as 

 to be considered almost homochronic. 



Contemporaneity of Faunas. That several distinct faunas may 

 exist side by side in not too widely separated districts is a well 

 known fact. The difference of faunas north and south of Cape 

 Cod on the Atlantic Coast may be mentioned as a modern ex- 

 ample ; also the difference between the Red Sea fauna and that of 

 the Mediterranean, and, finally, the distinct faunas on opposite sides 

 of the Isthmus of Panama. In all of these cases a partial or com- 

 plete land barrier separated the faunas. In the case of Cape Cod, 

 this barrier is incomplete and, although aided by cold currents from 

 the north, it has not entirely prevented the migration around it of 

 the faunas. The other two barriers were complete, and separated 

 faunas of different provinces, but the transsection in 1869 of the 



