110 THE AMERICAN NATURALIST [Vol. XLII 



consideration in the region of the globe which one is 

 studying. 



It is desirable to return to this general law of faunal 

 changes through migration and to illustrate its interest 

 more fully. 



The importance of the migrations of terrestrial animals 

 as correlated with great changes in the paleo geography 

 of the continents, was fully recognized a century ago by 

 Cuvier. The illustrious founder of paleontology had 

 been justly impressed by the absence or rarity of forms 

 of passage between the superposed fossil faunae. Exag- 

 gerating somewhat, owing to the imperfect evidence 

 before him, the consequences of this observed fact, Cuvier 

 had deduced from it the renewal of faunae in toto (after 

 their destruction by terrestrial cataclysms) not by suc- 

 cessive creations, as he is often accused of advocating, 

 but by extensive migrations of animals foreign to the 

 region. Since his time, many paleontologists, Wallace, 

 Lydekker, Zittel, Schlosser, Gaudry, Osborn, Ameghino, 

 etc., have given their attention to this subject and have 

 reiterated its significance. It appears to me, however, 

 that these contributions have been of too speculative a 

 character, inadequately supported by precise data. It 

 has resulted that the majority of the essays at the genetic 

 or structural phytogeny, which have been attempted in 

 various groups of fossil mammals, are defective, chiefly 

 because their authors have almost always sought to find 

 in place in the particular country which they inhabit the 

 various evolutionary series of these groups. 



No doubt there are great practical difficulties in re- 

 fastening link to link the segments of the broken chain 

 which forms the evolutionary series of each of the in- 

 numerable branches of the mammalia. Nevertheless, the 

 obstacles are smoothed away by each new discovery ; thus 

 the recent disinterment of the Oligocene and Eocene of 

 the Libyan desert, of the ancestors of the Proboscidea, 

 the Mastodons and Dinotherium, which appear so sud- 

 denly in Europe at the beginning of the Miocene, and 



