232 J. P. Smith — Periodic Migrations. 



former climate was uniform, it must have been warm, and the 

 temperature must have been warmer the further we go back 

 in geologic history. And yet we have evidence that the tem- 

 perature was not uniform even in the Paleozoic. It is well 

 known that over parts of India, Australia and South Africa 

 there is good evidence that there was a Permian glacial epoch, 

 while we know from abundant contemporary floras in other 

 parts of the earth that this glaciation was not universal. In 

 South America in the same latitudes as the glaciated region of 

 the Orient there does not appear to have been a cold period. 

 In this, then, we have proof that there was great diversity of 

 climate even in the Paleozoic era. 



Now it would be absurd to account for the difference 

 between the cold-water faunas of California in the Pliocene 

 and lower San Pedro formations and the warm-water fauna of 

 the upper San Pedro, and the contrast between the latter and 

 the present cooler water fauna of the California coast, on any 

 other hypothesis than differences of temperature. For all this 

 is based on species that are still living, where we know the 

 exact conditions of geographic connection and temperature 

 under which they live, and by which their distribution is 

 governed. .These differences of temperature are slight, and the 

 changes in physical geography that caused them are insignifi- 

 cant, though far-reaching in their effects. All this, of course, 

 applies only to the shore lines affected by the marine currents, 

 and does not necessarily have anything to do with continental 

 climates. 



It would seem equally rational to explain similar distribution 

 in the past by the same hypothesis. The faunal relations 

 between western America and eastern Asia from the Trias to 

 the present were the same, Asiatic facies alternating with 

 periodically recurring invasions of the Boreal type. If differ- 

 ences of temperature can account for the connections and 

 separations of the living faunas, they must be taken into 

 account in explaining similar connections and separations in 

 Tertiary, Cretaceous, and even Jurassic and Triassic times. 



On homotaxis. — The similarity of the fossil faunas of the 

 Orient to those of the west coast of America is, indeed, sur- 

 prising, but not more so than that of the living faunas of 

 Japan and the Calif ornian province. It is important in the 

 correlation of these deposits to determine whether they are 

 really synchronous. Ever since Huxley* cast doubt upon the 

 simultaneous occurrence of the same faunas in widely separated 

 regions, geologists have been inclined to assume that this simi- 

 larity is good proof that they were not really synchronous. 

 But we know that the present faunas of Japan and the Cali- 



* Quart. Jour. Geol. Soc, London, xviii, 40-54, 1862. 



