68 THE POPULAR SCIENCE MONTHLY 



elusions 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° P., 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 Surve} r , 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 Journal 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 



1 See especially Dall's " Contributions to the Tertiary Fauna of Florida," 

 published as Vol. III. of the Transactions of the Wagner Free Institute of 

 Science, Philadelphia, and a chapter in the Miocene volume of the Maryland 

 Geological Survey. 



