is,i Dickerson: A Fauna of the Vigo Group yj 
recognized only through comparative studies. These subspecific 
differences are exceedingly important for minute separation and 
discrimination of strata deposited under tropical conditions. 
FACTORS PROMOTING EVOLUTION OF PELECYPODS AND GASTROPODS 
The changes in conditions of environment of marine pelecypods 
and gastropods — salinity, temperature, depth of water, character 
of bottom, food, oceanic currents — determine the existence of 
individuals, and in all probability species also. Many marine 
forms are very delicately adjusted to their environment, and even 
slight changes may cause their extinction in certain localities. 
Of these conditions, change in temperature is probably the most 
important. The annual temperature range of waters in the 
Tropics is far less than similar ranges in the temperate zones. 
Likewise, variations in salinity are probably less, as this is a 
secondary factor dependent in large part upon temperature. 
The influence of oceanic currents is intimately connected with 
temperature and salinity. Depth of water and the character 
of the bottom may be altered by changes in the volume of sedi- 
ments brought into the ocean from the neighboring land and 
by epeirogenic (continent-building) movements which have 
caused a restriction or enlargement of a continental shelf. Epei- 
rogenic movements vitally affect food conditions of pelecypods 
and gastropods ; for, if the continental shelf is greatly reduced 
by uplift, the feeding areas are thus reduced, and if the com- 
petition among gastropods and pelecypods is too great, a species 
may rapidly become extinct. All of these changes are probably 
far less under tropical conditions than under temperate or arctic 
conditions. 
COMPARISON OF LIFE CONDITIONS DURING VIGO-MIOCENE TIME WITH 
THOSE OF RECENT TIME 
The close relationship between the Vigo-Miocene fauna of the 
Philippines and the Recent fauna of these same waters indicates 
that changes in living conditions since the beginning of Miocene 
time have been but slight. Apparently change in temperature 
has not had notable influence. It can be shown by general geo- 
logic evidence that an archipelagic condition existed during Mio- 
cene, Pliocene, and Pleistocene times. Salinity during the last 
half of the Tertiary and Recent has probably altered but little, 
and this only locally. Oceanic currents, and changes in character 
of ocean bottom were probably different during Vigo-Miocene 
time than during Malumbang-Pleistocene or Recent time, as is 
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