EVOLUTION AND DISTRIBUTION OF TERTIARY FAUNAS 215 



proportion of the total fauna will be represented, but these minor 

 groups are not entitled to the designation of a fauna as used in this 

 paper. 



Relations oj temperature to the fauna. — In considering the relations 

 of temperature of the water to the fauna, account must be taken of the 

 vertical differences. The temperature of the water at the surface 

 differs materially from that at the bottom in most regions, where the 

 depth is over a few fathoms. Arctic or Antarctic species may extend 

 in cold depths of ocean for thousands of miles; while, in the warm 

 superficial strata above them and inshore from them, a totally different 

 assembly lives and thrives. It is easy, in the case of widely diffused 

 northern species, when deep water dredgings have revealed the distri- 

 bution, to observe in the tables the boreal forms descending with the 

 temperatures to deeper and deeper water as they approach the tropics. 

 That this is so generally true is satisfactory evidence that the factor 

 of pressure, being equalized by thorough permeation of the organism, 

 is less effective in limiting distribution than most other factors. It 

 seems incredible that the large eggs of abyssal mollusks can go through 

 the processes of development under a pressure of tons to the square 

 inch; since there must be a limit somewhere to the permeability of 

 tissues. Still it is evident that they do. 



Why temperature should be so important in limiting distribution 

 is a question which may be answered in several ways. Brooks has 

 shown that, while the embryonic oysters {Ostrea virginica) are swim- 

 ming at the surface of the sea, an entire brood may be destroyed to the 

 last individual, by a fall in temperature of a few degrees, due to a cold 

 rain. While it is not improbable that oysters from the northern part 

 of the range of the species, say Nova Scotia, may have in the embry- 

 onic state a greater tolerance for a fall in temperature than those of a 

 relatively warmer region like Chesapeake Bay or the coast of Florida, 

 yet it seems likely that a certain narrow range of temperature is 

 required for the developmental stages, and that the distribution of the 

 species is limited to the area where such temperatures may be had 

 during the spawning season. 



Thus, for example, young Chesapeake oysters of an inch and a 

 half in breadth may be transported to the Pacific coast, planted in 

 suitable locations, and will flourish well, growing even faster than in 



