20 The Philippine Journal of Science 1921 
located in the Tropics offers a great variety of habitat and a 
new species entering such a region could on this account find 
suitable conditions for existence. 
From Miocene to Recent in the Tropics molluscan faunas have 
‘changed but little, and but slight specific alterations have 
occurred. Since a tropical or subtropical climate prevailed over 
California, Oregon, and Washington during upper Eocene time 
the great geographic and stratigraphic ranges of certain species 
of Tejon (upper Eocene) age are due to nearly uniform conditions 
of temperature and other factors mentioned. The great strat- 
igraphic range of many Tejon-Eocene species is probably due 
to uniformity in climate during long periods of time, and slight 
faunal changes have greater significance in the upper Eocene 
than corresponding changes in the Miocene, Pliocene, and Pleis- 
tocene time; these variations probably required a much longer 
time for their production as well. Uniformity in oceanic tem- 
perature enabled many species to range far to the north, and in 
fact far west of California to the Eocene of Japan where Peris- 
solax blakei, Pholadomya nasuta, or their near relatives 
occurred.® 
Eocene time then must not be measured by the same faunal. 
“vardstick” as Pliocene and Miocene time, but a much finer scale 
is required. It is the writer’s opinion, based upon the above con- 
sideration, that Eocene time is far longer than any of the other 
divisions of the Tertiary. : 
SUMMARY 
The tentative conclusion of the writer is that in the study of 
Tertiary faunas of the Tropics a different percentage scale must 
be used. For the later Tertiary, Miocene, Pliocene, and Pleis- 
tocene the percentages which apply in the temperate regions to 
the Pliocene are roughly adaptable to the Miocene; similarly, the 
percentages which apply in the temperate regions to the Pleis- 
tocene are apparently those of the Pliocene of the Tropics. This 
apparent lack of faunal differentiation during the Tertiary in 
the Tropics is due to uniformity of temperature, salinity, food, 
and other life essentials. From another viewpoint the rate of 
evolution of gastropods and pelecypods in the Tropics during 
the Tertiary was far less than during this same time in the more 
rigorous environs of the temperate zones. The tropical or sub- 
tropical faunas of the Pacific Coast of North America exhibit 
but slight differences compared to the faunas of Miocene and 
* Yokoyama, M., Some Tertiary fossils from the Miiki coal fields, 
Journ. Coll. Sci. Imperial Univ. of Tokyo, 27 (1911). 
