2i6 W. H. BALL 



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 

 some boreal elements. Thirty miles off shore, in less than fifty 

 fathoms, the fringe of the Gulf Stream protects a fauna in large part 

 identical with that which characterizes the Bahama Banks and 

 Bermuda. The large species of Venus, which penetrated to the north 

 shore of the Gulf of Mexico with the cool Miocene water, have 

 maintained themselves notwithstanding the subsequent rise of tem- 

 perature and persist in these new conditions to the present day, a 

 notable example of adaptation. On the other hand the subtropical 

 Rangia and Corbicula, which advanced with the warm Pliocene waters 



