EVOLUTION AND DISTRIBUTION OF TERTIARY FAUNAS 497 
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 
their native waters. Yet of the billions of spat which these oysters 
have discharged into the waters of the Pacific (fifteen or twenty 
degrees colder than the Chesapeake at spawning time) there is not a 
trace left in the shape of young oysters. In spite of the best efforts 
of the local oystermen the Chesapeake oyster has not become accli- 
mated. 
Another way in which temperature may affect a fauna is in pro- 
moting or inhibiting the minute plant-life which forms the food of 
many bivalves. In all cases it is certain that a fall below a certain 
level of temperature is more effective upon the animals subjected to 
it than a corresponding rise in temperature. The first, as I have 
indicated, may kill; the second, merely accelerate development. 
The very low temperatures nearly universal on the floor of the 
open ocean, and the otherwise uniform conditions that prevail there, 
offer favorable opportunities for wide distribution of boreal organisms. 
I am informed by Mr. A. H. Clark that the Antarctic Crinoidea, 
characterized by scaly segments, have penetrated by this road in the 
Eastern Pacific even to the Oregonian region; while on the opposite 
coast the smooth-segmented Arctic forms have been traced far to the 
southward. 
As indicators of subaerial conditions it is obvious that littoral 
invertebrates are more useful than those of deeper waters, since they 
are more exposed to climatic changes. It may happen that a vertical 
section of the submarine continental slope drawn at right angles to 
the coast from the shore to the oceanic floor may, and in most cases 
will, cut through a series of different faunas corresponding to the 
temperatures encountered. Off Cape Hatteras the cold inshore 
current from the north is the haunt of a cool-temperate fauna with 
