RELATIONS TO OTHER SCIENCES 555 



of life arose and dispersed. The oldest known fossils, however, occur 

 in rocks at the base of the Cambrian series, both in the tropics and in 

 the far north; and there is as yet no means of determining whether 

 the animals represented by these fossils spread from the north, south, 

 or equatorial regions, or from several points. There is thus no direct 

 evidence from fossils for or against the theory of the polar origin of 

 life. The facts supposed to show that the same area continued to be 

 a source of new organisms even until the later Tertiary period admit 

 of other interpretations which are in better accord with the newest 

 discoveries. 



Even of changes which may have occurred since the globe became 

 habitable, fossils furnish no reliable indications. Professor G. H. Dar- 

 win's theory of the former magnitude of the tides is as completely 

 unsupported by paleontology as by geology. The idea that the 

 earth's atmosphere has gradually altered in constitution since life 

 began is equally destitute of support from fossils. The microscopical 

 structure of the leaves of the Carboniferous plants suggests that even 

 at so remote a period as that when they flourished, the air was essen- 

 tially identical with that of the present day, without any superfluity 

 of carbon dioxide or anything to obstruct the full influence of the 

 sun's rays. 1 



Relations to Geography 



So far as can be judged at present, paleontology justifies the 

 assumption that each type of animal or plant has only originated 

 once and from one set of ancestors. Fossils can therefore be used as 

 an aid to the solution of geographical problems. If a more or less 

 sedentary group of animals is found to be essentially identical in two 

 widely separated seas, it may be reasonably assumed either that those 

 seas were once connected, or that they received their life from a com- 

 mon source. Similarly, if two distant tracts of land are inhabited by 

 the same animals and plants, and there is no possibility at present 

 of migration between these two regions, a former connection either 

 with each other or with a common centre may also be postulated. 

 The same is true in reference to all periods of the earth's history, and 

 hence the varying distribution of fossils at different epochs affords 

 a clue to the successive changes in the disposition of lands and seas, 

 gradually culminating in their present arrangement. 



For instance, it has been lately noticed 2 that the mollusca living 

 on the two opposite coasts of the North Pacific during the Pliocene 

 period were much more nearly identical than they are at the present 



1 A. C. Sewurd, Fossil Plants as Tests of Climate (Sedgwick Prize Essay, 1892), 



* R. Arnold, The Paleontology and Stratigraphy of the Marine Pliocene and 

 Pleistocene of San Pedro, California. Memoirs of the Calif. Acad. Sci., vol. m 

 (1903). 



