. The Philippine 
Journal of Science 
Vol. 18 JANUARY, 1921 No. 1 
A FAUNA OF THE VIGO GROUP : ITS BEARING ON THE 
EVOLUTION OF MARINE MOLLUSCAN FAUNAS 
By Roy E. Dickerson 
Honorary Curator, Department of Paleontology, California Academy of 
Science, Golden Gate Park, San Francisco, California 
TWO PLATES 
A comparison of the rate of evolution of a marine invertebrate 
fauna in the Tropics with that of faunas of the temperate zones 
brings out some interesting results. During the past year, 
1919-1920, the writer has had the opportunity and rare good* 
fortune to collect some excellently preserved fossils from thle 
Vigo group of the Philippine Islands, incidental to some eco^ 
nomic investigations in which he was engaged. After several 
years spent in study of the faunal Tertiary problems of the 
Pacific Coast of North America, the writer naturally had ac- 
quired a point of view of the worker in temperate climes to 
a certain extent. However, many interesting problems in the 
Eocene of California, Oregon, and Washington suggested that 
marine Eocene molluscan faunas did not evolve as rapidly as 
those of the Miocene and Pliocene and that the same “yard 
stick” in the Tertiary geological time scale could not be applied. 
Many problems of the Eocene are directly connected with the 
rate of evolution of a tropical fauna and as the Eocene faunas 
of California, Oregon, and Washington are essentially tropical 
or subtropical, the writer was glad to devote spare time to the 
study of a tropical fauna. 
Does the Lyell percentage system apply to tropical inverte- 
brate faunas? In answering this question one must bear in 
mind that this scale is really an expression of the time rate of 
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