516 THE AMERICAN NATURALIST [Vol. XLIII 



ker 12 wrote his work on the Geographical History of Mam- 

 mals. He was under the impression, therefore, that the 

 mammalian fauna of the South American region had been 

 totally isolated from that of North America up to about 

 the end of the Miocene. I shall shortly return to Pro- 

 fessor Osborn's views as soon as I have completed my 

 brief historical review of the first problem. 



Professor Lapparent 13 concurs with Mr. Lydekker's 

 opinion that the interchange of waters between the At- 

 lantic and Pacific oceans across Central America could 

 only have ceased to exist at quite the end of the Miocene 

 period. 



Finally, in his treatise on the development of conti- 

 nents, Dr. Arldt 14 maintains that the Central American 

 land bridge must have originated at the commencement 

 of the Pliocene period, and with this view I fully agree. 

 Part of Central America no doubt had already risen 

 above the ocean at a much earlier period, but in its present 

 outlines and extent it must be regarded as a geologically 

 recent development. 



All those in fact who have seriously considered the 

 problem, either from the standpoint of a marine or a ter- 

 restrial zoologist or from that of a paleontologist concur 

 in the opinion that North and South America were sepa- 

 rated from one another by a marine channel or by wide 

 seas during part of the Teritary era. This, however, is 

 the oj^j point in which there is a general agreement. 

 While some contend that the junction between the two 

 continents had only been effected in comparatively recent 

 geological times, others hold that within the life history 

 of the great class of mammals, either in the early Tertiary 

 or late in the Secondary era, a land bridge between North 

 and South America had once before existed, by means of 

 which an interchange of the faunas could have been 

 brought about. It is this supposed earlier land connec- 



13 Lydekker, B., ' < Geographical History of Mammals," p. 119- " 

 13 Lapparent, A. de, " Traite de Geologie," p. 1318. 

 "Arldt, Th.. "Eiihvickhmg dor Kontinente, " p. 597. 



