22 CALIFORNIA ACADEMY OF SCIENCES TProc. 4th Se». 



existence of individuals, and, in all probability, species also. 

 Many marine forms are quite delicately adjusted to their en- 

 vironment 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 the 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 bottom and 

 the character of the bottom may be altered by changes in the 

 volume of sediments brought into the ocean from the neighbor- 

 ing land and by epirogenic — continent building — movements 

 which have caused a restriction or enlargement of a continental 

 shelf. Epirogenic 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 competition 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 the Recent 



The close relationship between the Vigo Miocene fauna of 

 the Philippines and the recent fauna of these same waters, indi- 

 cates that change in living conditions since the beginning of 

 Miocene time has been but slight. Apparently change in tem- 

 perature has not had any notable influence. It can be shown 

 from general geological evidence that an archipelagic condition 

 existed during Miocene, Pliocene and Pleistocene times. Salin- 

 ity 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 evidenced by the absence of reef building 

 corals from the Vigo and the argillaceous and sandy character 

 of the sediments of this group. A study of Vigo sedimentation 

 indicates that an extensive land area stretched from north to 



