Apkil 13, 1900.] 



SCIENG.E. 



563 



Sclatei', Allen, Newton and Blanford, 

 working upon living birds and mammals, 

 continued this investigation, but it re- 

 mained for Blanford, in 1890, to prove that 

 the world zoologically should be divided 

 into three great divisions ; an Australian, 

 a South American and a third region, Arc- 

 tog£ea, comprising ISTorth America, Europe, 

 Asia and Africa. 



jSTow it is clear that exactly as our under- 

 standing of the relations of living animals 

 and plants to each other depends upon 

 their fossil ancestors or upon their paleon- 

 tology, so the final test of a scheme of 

 zoological distribution must be a paleon- 

 tological test. The animals of various 

 families and orders have either originated 

 in or migrated into their present habi- 

 tat in past time, so that the geological 

 record as to their order of appearance be- 

 comes of first importance. Here again the 

 necessity of an absolutely reliable correlation 

 time scale such as we are now establishing 

 becomes evident, for the very first step 

 toward an exact solution of the problem of 

 past migration is to establish, as far as pos- 

 sible, the faunal parallels upon different 

 continents, we can then determine where 

 certain types of animals first appeared, and 

 distinguish between the autocthonous en- 

 demic or native types and the migrant or 

 new types. 



Tliis then is our problem, to connect living 

 distribution xuith distribution in past time and 

 to propose a system ivhich will be in harmony 

 with both sets of facts. 



The tests of synchronism between Euro- 

 pean 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 predomi- 

 nance 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 ap- 

 parently 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 (Pro- 

 boscidia) in Europe and America. 



Unfortunatelj' there is still no agreement 

 among zoologists as to the faunal geograph- 

 ical divisions. Lydekker, well versed in 

 both paleontology and zoology, has for the 

 first time brought together both classes of 

 evidence in his recent valuable work upon 

 the ' Geographical Distribution of Mam- 

 mals '; he shows conclusively that zoo- 

 paleontology favors the division of the 

 world into three great realms as proposed 

 by Blanford, to these may be applied the 

 terms Arctogsea, ISTotogsea and Neogsea, as 

 proposed anonymously in 1893. (Chart 

 II.) 



Geographically, these realms are con- 

 nected by low lying portions of the earth, 

 which, duriug long periods of submergence 

 beneath the sea, have completely isolated 

 them. At the same time we are forced to 

 conclude that there were shorter intervals 

 of elevation or land continuity at various 

 times during the Tertiary period. 



ISTow it is a well-known principle of zoo- 

 logical evolution that an isolated region, if 

 large and sufficiently varied in its topog- 

 raphy, soil, climate and vegetation, will 

 give rise to a diversified fauna according 

 to the laiv of adaptive radiation^ from primi- 

 tive and central types. Branches will spring 

 off in all directions to take advantage 

 of every possible opportunity of securing 

 food. The modifications which animals 

 undergo in this adaptive radiation are 

 largely of mechanical nature, they are lim- 



*So termed by the writer, see ' Rise of IMammalia 

 in North America,' 1893, and 'Origin of Mammals,' 

 1898. 



