3IATTHJJW. CUM ATI-] AND J:\OLUTION !;(! 



(1778) by Buffon.*^ AVitli a clearer perspective of geologic time and far 

 more exact records, it is clear that most of this deployment and dispersal 

 of the mammalian races has taken place since the Eocene epoch of the 

 Tertiary, altliough remnants of an older dispersal on the same lines are 

 probably traceable in the present habitat of monotremes, marsupials and 

 primitive insectivores. 



INTEEPllETATION OF SUPPOSED EXCEPTIONS 



There has been a disposition in recent years among students of geo- 

 graphical distribution to lay weight upon certain apparent exceptions to 

 this general rule, where the geological record has not yet afforded evi- 

 dence to support the northerly origin of certain groups now limited to 

 the southern continents or to the tropics and to infer various equatorial 

 or southern continental connections during or previous to the Tertiar\% 

 in order to account for these exceptions." To these hypotheses, there are 

 several objections : 



1) The evidence for the general permanence of the great ocean basins and 

 tlieir maintenance formerly, as now, by isostatic balance is very strong and 

 direct, and before allowing any exceptions, we should be very sure that no 

 other explanation will serve. 



2) The instances adduced in favor of former equatorial or southern con- 

 nections are distinctly "exceptional cases in the faunte, which may, in all the 

 cases I have examined, be accounted for by appealing to the imperfection of 

 the geologic record, by parallelism or by the rare accidents of over-sea trans- 

 portation, 



3) The existence of such land bridges would present the opportunity for 

 migration of other parts or of the whole of certain faunte, which has evidently 

 not occurred. I can see no good reason wh^ the only animals which availed 

 themselves of such continental bridges should be the ones which might be 

 accounted for in other ways, while those which would furnish conclusive proof 

 are invariably absent. 



^ See K. V. Zittel, History of Geology and I'ahvontology, p. 43. for a brief summary of 

 Bnffon's views on tliis subject. The tlieory has been more fully presented by many sub- 

 sequent writers. Tn recent years, it has been very ably set forth in its relations to 

 Tertiary mammalia by Dr. J. L. Wortman (Amer. Jour. Sci.. 190.3). A very readable 

 little pamphlet by G. Hilton Scribner. entitled "Where Did Life Begin". 1884. while 

 totally deficient in geological perspective, sets forth very clearly the diverse effect upon 

 migration of the general trend of the great mountain system, north and south in the 

 New World, east and west in the Old. Alfred Russell Wallace is. I believe, usually 

 regarded as the foremost exponent of this theory on the distributional side: but it is 

 scarcely necessary to catalogue the principal exponents of a view so long and so gen- 

 erally held. 



^ The distinguished Argentine paleontologist. Florentino Ameghino, has for twenty 

 years past advocated a theory the direct opposite to that currently held, and he would 

 derive practically all groups of mannualia from a South American center of disi)ersal. 

 The evidence foi' and objections to this theory will he discussed in the sequel. 



