EVOLUTION AND DISTRIBUTION OF TERTIARY FAUNAS 495 
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 
winds from a quarter which led to the forcing to the eastward of the 
Gulf Stream water, and to the influx of much colder water from the 
Polar current into the area thus vacated, was believed to be respon- 
sible for the almost total extermination of these fishes, which were 
found floating dead and apparently uninjured in millions on the 
surface of the sea, by navigators bound into New York and adjacent 
ports. 
This temperature change which lasted at most for a few weeks 
would probably have had no effect whatever on the adult larger 
invertebrates of the same area, though to any of their larval young it 
might well have proved fatal. Another season would replace these, 
but the restocking of the fauna with “tilefish,’ which finally took 
place, required many years. 
A statement of the factors which are regarded as modifying 
existing marine invertebrate faunas will put the student in possession 
of the chief factors which may have affected analogous faunas during 
past geologic time. My point of view is that afforded by a knowledge 
of conditions affecting molluscan life. 
Census oj species—From a discussion too long to quote here in 
full, I have drawn the following conclusions: That the part of the 
average mollusk-fauna which is capable of leaving traces in the shape 
of fossils, under conditions not greatly differing from those of the 
present day, in a region where the temperature of the sea ranges 
during the coldest winter month between 32° and 40° F. (which 
might be called boreal), would comprise about 250 species. In case 
the temperature ranged between 40° and 60° (cool temperate) about 
1 Bull: U. S. Geological Survey, No. 84, Correlation Papers, Neocene, 1892, pp- 
25-28. 
