494 W. H. DALL 
question was, when living, subject to the action of an analogous 
environment. 
To illustrate this second proposition it may be said that if fig trees 
can now flourish and reproduce their species only in regions having 
a mean minimum temperature of thirty degrees Fahrenheit,iand a 
summer mean temperature of not less than sixty degrees; . and, 
secondly, if we find in the Tertiary leaf-beds of Greenland and 
Spitsbergen indications of groves of fig trees having flourished there 
in the Oligocene epoch; then we are likewise justified in assuming 
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 lability 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- 
