496 WHEELER. 



geologic record, mere negative evidence is not sufficient to cause them 

 to be set aside. 



"2. No account is taken of faunal interchanges often much more 

 extensive, which would presumably have taken place if the land- 

 bridges assumed had existed, but which have not taken place. It 

 may here be urged that this too is negative evidence. But the nega- 

 tive evidence derived from an appeal to the geological record is weak, 

 not per se, but because of the demonstrated imperfection of the record. 

 On the other hand, there are many instances where a land-bridge is 

 well proven, and in these cases it is not a few scattered exceptions, but 

 an entire fauna that has migrated, subject only to the restrictions 

 imposed by climatic or topographic barriers of other kinds." 



In accounting for the present discontinuous distribution of many 

 ancient and primitive forms Matthew seems to me to have made good 

 use of a principle which seems to have been first suggested by Haacke 

 (1887). This writer called attention to the fact that at the present 

 time the most primitive types of the various groups of animals are 

 mostly confined to the tropics and the southern hemisphere. This 

 can be most readily explained on the supposition that the situations 

 in which such forms now live are not their original habitats but those 

 to which they have been relegated by more recent and more specialized 

 forms evolving and usurping their places in the territory originally 

 occupied by the group. Hence the oldest and most primitive members 

 of a group come to be found today at the periphery of its range and 

 the more recent and specialized forms in or near its center. Clark 

 (1915) has reached the same conclusion from his study of the dis- 

 tribution of the Onychophora. He says: "Any animal type, once 

 evolved, will extend itself immediately in every direction as far as the 

 natural barriers to its dispersal; a more specialized form (a dominant 

 type) of the same animal, better fitted for the conditions under which 

 it lives, will sooner or later be evolved somewhere in the central, or 

 more favorable portion of the territory inhabited by the original 

 type; this new type will at once extend itself as did the original type; 

 but in the meantime there may have arisen certain barriers, which the 

 second type cannot cross and beyond which, therefore, the first type 

 is secure. Up to these barriers — high mountains, deserts, newly 

 formed arms of the sea, or whatever they may be — the second type 

 will gradually supplant the first, as a result of its better economic 

 equipment and more perfect physical resistence, and the advantage 

 which this better equipment and resistance give it in the struggle for 

 existence. Thus we shall eventually find a specialized type beyond 



