224 GEOLOGICAL DISTRIBUTION. 



The assumption that because certain marine genera, of whose habits 

 we know nothing, are found in one region and not in another there 

 must of necessity be well-marked differences in the thermometric 

 conditions of the waters which they inhabit, is at best a precarious 

 one, inasmuch as it is founded almost exclusively on negative evi- 

 dence. This is clearly proved by the status of the Indian Jurassic 

 fauna, where, as we have already seen, one of the distinctive "Medi- 

 terranean '' genera of ammonites, Phylloceras, is abundantly inter- 

 mingled with those supposed to represent a more northern facies, 

 while the other, Lytoceras, is almost altogether absent. If it be 

 assumed that the presence of Aucellee in the Himalayafauna suffi- 

 ciently demonstrates the cold-water facies of that fauna simply 

 because the genus is most abundantly developed in the northern 

 regions. Central and Northern Bussia, Siberia, Spitzbergen, and 

 Greenland, why may it not just as well be assumed that the 

 northern fauna was of a warm-water facies, from the fact that the 

 same genus is abundantly represented in a region lying on the im- 

 mediate confines of the tropics ? The great north and south range 

 of the genus Aucella, which is also found along the thirty-seventh 

 parallel of latitude in California (Cretaceous), far from proving the 

 existence of homoiozoic belts, indicates rather the contrary. 



But very little dependence can be placed upon the genera of 

 Mollusca as indicating the thermal conditions of the waters which 

 they inhabited. There is at the present time on the east coast of 

 the United States, south of the forty-first parallel of latitude, only 

 one species for each of the three boreal, or Arctic (so recognised), 

 genera Astarte, Leda, and Nucula. Yet, during the Eocene and 

 Miocene periods, these same genera were abundantly developed in 

 the waters situated seven and ten degrees of latitude farther to the 

 south, and where, consequently, we might conceive a stratum of 

 cold water to have existed. But, unless certain astronomical or 

 physical conditions prevailed at the time with which we are not 

 acquainted, it is practically certain that the temperature of the water 

 was at least as high, if not higher, than it now is. Nor can it be 

 urged that in these Tertiary deposits we are dealing with a deep-sea 

 (cold water) fauna, or that a low temperature might have prevailed 

 simply as the result of the non-existence of a Gulf Stream — a con- 

 tinuous sea separating the North and South American continents — 

 since we have just as conclusive testimony favouring the supposition 



