THE DEVONIAN SECTION OF ITHACA, N. Y. 597 



tinguishing their characteristics is greater. Clarke has recognized 

 this difficulty in saying that 



it is extraordinarily difficult to fix on a division plane between the Ithaca and 

 the overlying Chemung faunas, as the one passes into the other by easy grada- 

 tion, and 'we are still somewhat at a loss in determining specific values indicial 

 of the early stages of Chemung time/ 



This difficulty has been met in the present revision by recognizing 

 the reasons for the difficulty and drawing the lines accordingly. The 

 reason why it is difficult to draw a stratigraphic horizon plane, 

 separating two faunas in which a large part of the genera are iden- 

 tical and many of the species the same, is because of the evident 

 similarity in composition of the two faunas. This similarity, I 

 have assumed, is due to adjustment of the species of the faunas to 

 like conditions of environment, but in different areas of distribution. 

 The fact that the two faunas contain some distinctive species, asso- 

 ciated with a second fact that in the New York province at least 

 the one (Chemung) is always found higher in any single section than 

 the other (Ithaca) fauna, together prove that distinguishing char- 

 acters are of long standing, From this fact it is inferred that the 

 two faunas developed their peculiarities in different areal centers. 

 The differences are like those distinguishing the Arctic from the 

 Florida faunas of the western Atlantic, or the faunas of the east coast 

 of the United States of America from those of the coast of Europe. 

 ' If these interpretations are correct, two kinds of differences should 

 be capable of discovery, viz.: (i) differences in the evolution of the 

 species of a common race, and (2) differences in the original stock 

 of the two faunas, i. e., survivals of old races which have become ex- 

 tinct in one and have continued to live in the other. 



If we attempt to draw the line of boundary between the two 

 faunas, stratigraphically, it is necessary to assume that the later 

 fauna is not the strict genetic successor of the earlier one, in spite of 

 the close similarity in its species. Either barriers have been removed 

 opening to a common basin the waters of separate basins, or general 

 shifting of temperature by currents or by depth has produced such 

 changes of general temperature that the two faunas have been forced 



I "Naples Fauna of Western New York," New York State Museum, Memoir 

 No. 6, Part 2, p. 213 (1903). 



