18,1 Dickerson: A Fauna of the Vigo Group 19 
to its environment, is not apparently present in the case of the 
tropical pelecypods and gastropods, and the slow changes of 
fauna are apparently produced by slight changes in temperature, 
depth, salinity, and food. In other words, the “wonderful sta- 
bility of protoplasm” seems to be exhibited in these marine trop- 
ical invertebrates, except when environmental changes impress 
alterations upon this vital life substance. 
CROWDING OF SPECIES AND THE RECENT FAUNA OF THE PHILIPPINES 
A seeming objection to the main thesis of this paper is found 
in connection with the great abundance of species in the marine 
waters surrounding the Philippine Islands. As was mentioned 
above, climatic zones were by no means as sharply differentiated 
during the early portion of the Tertiary as during the later. 
Practically all Recent tropical genera were initiated in the 
Eocene, and many of the species representing these genera had 
during this period an exceedingly wide geographic range, par- 
ticularly as respects latitude. During this time tropical species 
flourished in high latitudes. To use a simple comparison, the 
tropical life “accordion” was extended to its greatest limit. The 
exact nature of the change which caused a separation of remark- 
able distinctness between the Oligocene and the Eocene faunas of 
the Pacific Coast of North America is not fully understood. It 
seems probable, however, that the time represented by uncon- 
formity between Oligocene and Eocene was long. The distribu- 
tion of land masses on the earth was profoundly affected, and 
it seems quite probable that the climate during this ep-Eocene 
time was decidedly cooler than in the Eocene or the Oligocene 
which followed. It seems quite probable that the life ‘‘accordion” 
was compressed, and that many species which ranged far to the 
north in Eocene time were compelled to seek the more genial 
climates of the tropic seas. When the faunas during the Oligo- 
cene again had a chance to expand into higher latitudes, they 
encountered new conditions of environment and were nearly all 
specifically changed. The Oligocene faunas of Oregon, Washing- 
ton, and California are distinctly set off from the Miocene, and 
similar changes may have taken place during ep-Miocene time. 
Again, many of the species succeeded in making a strategic 
retreat. Even more pronounced were the “accordion”-like 
changes during the Pliocene and the Pleistocene. 
As was pointed out above, archipelagic conditions prevailed 
in the Philippines during the Tertiary, although the record for 
the Eocene is missing, or extremely meager. An archipelago 
