2;)2 AWALS M:\V YOh'h ACADEM) OF fiClENCES 



tion of their source and the history of their migration and dispersal. 

 Absolute standards, as of world-wide changes in physical or climatic con- 

 ditions, may serve in the future to give us broad lines of correlation inde- 

 pendent ot paleontology ; but at present their universality is hypothetical, 

 the exact train of physical ])lienomena wliich they entail and the indices 

 by which they may be recognized in the ^ratigraphic succession are im- 

 perfectly known. Paleontology is for the present our sole recourse in 

 correlation. Probably it will always be our chief dependence, at least in 

 exact ami detailed comjjarison. 



SYN^CItttOXtSM AND IIOMOTAXIS 



The ordinaiT metliods of paleontologic correlation can l)e applied with 

 arcuracy and certainty only over limited areas of the earth's surface. 

 When a])plied to far-distant regions, we meet first with the difficulty that 

 there is little identity of fauna?, only an equivalence more or less exact. 

 Nor can we be sure that equivalent or even identical forms were contem- 

 poraneous in all parts of the earth. They certainly are not so to-day. 

 The modern land fauna of Australia, as Huxley long ago insisted,^" is in 

 its broad lines a Mesozoic fauna. Examined in detail, it shows indeed 

 the marks of a long period of independent evolution and specialization. 

 Yet the degree and amount of specialization is fur less than that which 

 the fauna? of the northern continents have undergone during the Ceno- 

 zoic. The modern fauna of the East Indies or of Central Africa has a 

 great deal in common with the later Tertiary faunas of Europe and north- 

 ern Asia. Central America and tropical South Amerira bear similar 

 relations to Xorth America. While Huxley's dictum thai an older fauna 

 in one region may be homotaxial with a later fauna in another does not 

 apply to the extent of involving identity of all or most of the species, yet 

 it very clearly does ap])ly in a broad way to the land faunns and probably 

 to a less extent to the marine faunae as well. The rate at which evolution 

 and differentiation progress varies as between the fauna? of different re- 

 gions. It varies as between the different constituents of a fauna. Neither 

 the partial identity nor the general equivalence of two fauiKV is sufficient 

 to prove them synchronous, except under certain conditions to be con- 

 sidered later. 



Another method very generally used in correlation of fauna? which 

 contain little or nothing in common consists in an estimate of their rela- 

 tive antiquity as indicated by the proportion of extinct to surviving spe- 

 cies or genera. This also involves the assumption that the rate of progress 



"T. H. Huxley: Q. I. G. S., vol. sviii, pp. xl-liv. 18G2. 



