EVOLUTION AND DISTRIBUTION OF TERTIARY FAUNAS 213 



that in Greenland at that epoch the summer mean temperature did not 

 fall far below sixty degrees, nor the winter cold maintain itself greatly 

 below the minimum above mentioned. 



Among marine animals a consensus of the evidence on record 

 points insistently to temperature as the most important factor in 

 determining the existence and persistence of species in a given area; 

 and the toleration oi an organism and its progeny for fluctuations 

 of temperature limits its geographical distribution as positively as 

 would a material barrier. In the absence of such mortal extremes 

 of temperature, material barriers, unless hermetically complete, 

 really count for very little in determining distribution. 



In utilizing fossil faunas as chronologic indicators of geologic 

 time, the marine faunas are more readily utilized for the grand 

 divisions of the scale than the land faunas, especially when the latter 

 are characterized chiefly by fossil vertebrates. This is because the 

 marine conditions are more uniform, less affected by meteorologic 

 factors, and more dependent upon conditions which affect the whole 

 hydrosphere rather than small areas of it. The struggle for life is 

 less intense, the food supply generally more adequate, enemies less 

 vigorous, and dangerous fluctuations of temperature far less frequent, 

 in the sea than on land. 



The same features make the land faunas more clearly indicative 

 of minor divisions of the scale, and of the progress of organic evolu- 

 tion in the general region concerned; while less conclusive as to the 

 contemporaneity of widely separated though analogous faunas. 



The liability to sudden extermination by epidemic diseases, or 

 by sharp meteorologic changes of very short duration, or even by the 

 incursion of multitudes of small enemies, insects, or carnivora injuri- 

 ous to the young, is vastly greater among the land vertebrates than 

 among marine animals. Marine vertebrates are more subject to 

 injury from temporary causes than are the invertebrates associated 

 with them. A marked instance of this was the destruction of the 

 "tilefish" of the middle Atlantic coast a quarter of a century ago, if 

 the explanation finally accepted as most probable by Professor 

 Baird and other experts be the true one. The "tilefish" inhabited 

 a region where the water, warmed by the proximity of the Gulf 

 Stream, was of a moderate temperature. The combination of violent 



