272 ANNALS NEW YORK ACADEMY OF SCIENCES 



America. It is displayed there quite as clearly in races, such as the crice- 

 tine rodents, cervidae, etc., which are admittedly of Northern origin, as it 

 is in any autochthonous groups. Hence, it cannot be attributed to a gen- 

 eral Antarctic dispersal center, but must be explained as a parallel evolu- 

 tion under similar climatic stimulus. 



The general distribution of Mammalia dn.these lines is almost univer- 

 sally accepted; but many writers have pointed out certain supposed ex- 

 ceptions and found it necessary to account for them by various hypo- 

 thetical continental bridges. A careful consideration of these supposed 

 exceptions shows that, if due allowance be made for parallelism and for 

 the imperfection of the record, each one can be more satisfactorily inter- 

 preted in accordance with the general law. And the acceptance of any 

 such continental bridges would entail migrations of other groups wliich 

 assuredly have not occurred. The hystricomorph rodents of South Amer- 

 ica afford a single exceptional instance, in which over-sea transportation 

 from Africa appears to be the only reasonable interpretation of the evi- 

 dence at hand. 



I place much greater weight on the evidence from mammalian distri- 

 bution than on that of any other terrestrial group for several reasons, as 

 follows : 



1) Their past history, the time, place and method of evolution of the vari- 

 ous races, is better known than in any other group of land animals or plants. 



2) The complexity of structure in the hard parts which are pi-esoi'ved as 

 fossils is greater, affording a larger amount of evidence by which we may dis- 

 tinguish parallel or analogous races and determine the closeness of their real 

 affinities. As Stehlin"^ has recently observed, a single tooth of a mammal 

 affords as much structural evidence whereby to determine its relationships as 

 the entire skeleton of most invertebrates. Where our evidence is thus lim- 

 ited (to a single tooth, for example), we may, and frequently do, find difficulty 

 in deciding the exact affinities of a fossil mammal. But where we have the 

 skull or the skeleton or even the entire dentition, the results are correspond- 

 ingly sure and precise as the data are more extensive. 



3) Owing to their nearness to ourselves, their large size and other causes, 

 we are better able to understand their adaptation and observe and appreciate 

 the factors which may affect their evolution and migration. 



In dealing with the evidence furnished by the lower vertebrates and 

 invertebrates, we are hampered by the wider limits of time within which 

 the migration may have taken place, by the relative simplicity of the 

 structure of the hard parts, which makes it less easy to distinguish paral- 



"8 "tjber die Saugethlere der Schwelzerische Bohnerzformation." Verb. Schw. Naturf. 

 Gesell., 93 Jahresvers. 1910, Basel, r. 11 of separate. 



