68 THE POPULAR SCIENCE MONTHLY 
clusions that may be drawn from them vary greatly in positiveness. 
To take a familiar example, the reef-building corals are now restricted 
to shallow waters in which the mean temperature during the coldest 
month in the year is not less than 68° F., and such conditions are not 
found in the northern hemisphere north of latitude 32°. Since late 
Tertiary corals differ but little from those of the present time it is 
justifiable to assume that coral reefs in late Tertiary rocks indicate 
waters of about the temperature stated. But when Jurassic coral reefs 
are found as far north as latitude 53° it is by no means so certain that 
they indicate a minimum monthly mean temperature of 68° F., and 
concerning Devonian and Silurian coral reefs in high latitudes the 
doubt must be still greater. At the present time large reptiles are 
mainly confined to hot moist climates, but that fact alone can not be 
considered proof that the Mesozoic dinosaurs required the same kind 
of a climate. 
The impress of climate on the present fauna is shown in various 
ways. A tropical fauna contains the greatest number of species and 
exhibits its luxuriance in other ways. Thus, taking shell-bearing 
marine mollusks to illustrate the general law, Dall has shown in Bul- 
letin 84, U. S. Geological Survey, that the average tropical fauna in 
shallow waters consists of over 600 species, while the temperate fauna 
has less than 500 species, and the boreal fauna only 250. Again, there 
are certain genera that are characteristic of particular zones, and as- 
semblages of forms that are recognized as belonging only to frigid, or 
temperate, or tropical waters, and in genera that have a wide range 
many of the species are restricted to certain limits of temperature. 
In the late Tertiary faunas which contain a large proportion of 
living genera and many living species justifiable inferences as to climate 
may be made from direct comparison with living faunas. By one or 
another of the tests just indicated, or by a combination of them, Dall 
has produced convincing evidence that the Oligocene fauna of the 
Atlantic states was subtropical and that the Oligocene maintains its 
subtropical character even as far north as Arctic Siberia. He has also 
shown that the Miocene fauna of Maryland indicates a temperate 
climate and that a similar cool-water fauna extended at that time as 
far south as Florida.1 The fossils of the raised Pliocene beaches at 
Nome, Alaska, according to the same investigator, furnish evidence of 
warmer climate during Pliocene time even at that high latitude. By 
similar methods, in a paper published in the J ournal of Geology, Vol. 
XVII., Arnold has recently argued for a series of climatic changes in 
the late Tertiary and Pleistocene of California. 
When the investigation is carried back to the Mesozoic and earlier 
1See especially Dall’s “ Contributions to the Tertiary Fauna of Florida,” 
published as Vol. IIL. of the Transactions of the Wagner Free Institute of 
Science, Philadelphia, and a chapter in the Miocene volume of the Maryland 
Geological Survey. 
