48 OSBORN. 



TJiis thill is our problem, to connect living distributioji with dis- 

 tribution in past time and to propose a system which will be in 

 harmony with both sets of facts. 



The tests of synchronism between European and American 

 depositions are four-fold : First, the presence of a number of 

 identical or closely allied genera and species. Second, similarity 

 in the steps of evolution in related animals. Third, the pre- 

 dominance and spread of certain animals, as of the odd-toed 

 Ungulates in the middle Eocene and of the even-toed Ungulates 

 in the Upper Eocene. Fourth, the sudden appearance of new 

 types which have apparently originated elsewhere and have en- 

 joyed an extensive migration, so that they appear simultaneously 

 in different regions of the earth. An instance of this kind is 

 afforded by the unheralded appearance of new types in the base 

 of the Oligocene (Rhinoceroses) and of the Miocene (Probos- 

 cidia) in Europe and America, (See Part I, pp. 22 and 26.) 



Unfortunately there is still no agreement among zoologists as 

 to the faunal geographical divisions. Lydekker well versed in 

 both palaeontology and zoology, has for the first time brought 

 together both classes of evidence in his recent valuable work 

 upon the *' Geographical Distribution of Mammals," he shows 

 conclusively that zoo-palaeontology favors the division of the 

 world into three great realms as proposed by Blanford ; to these 

 may be applied the terms Arctog.ea, NotoG-^a and NEOOiEA, 

 as proposed anonymously (Sclater) in 1893.^ (Fig- i-) 



Geographically, these realms are connected by low lying por- 

 tions of the earth, which, during long periods of submergence 

 beneath the sea, have completely isolated them. At the same 

 time w^e are forced to conclude that there were shorter intervals 

 of elevation or land continuity at various times during the Ter- 

 tiary period. 



Now it is a well-known principle of zoological evolution that 

 an isolated region, if large and sufficiently varied in its topog- 

 raphy, soil, climate and vegetation, will give rise to a diversified 



^ In a review of papers by Merriam and Allen (The Nearctic Region and its 

 Mammals, Natural Science, 1 893, p. 289), P. L. Sclater observes " Thus we have a 

 very obvious threefold division of the earth's surface, taking mammals as our text, 

 into what may be called Notogaea, Neogaea, and Arctogaea.^'' 



