250 CARNEGIE INSTITUTION 



east and west direction, but subject to deflection northward by Iow> 

 hot plains, and southward by mountain ranges ; also that in the 

 ascent of a mountain rising from the area occupied by one of the 

 more southerly life zones, assemblages of animals and plants closely 

 corresponding with those of the more northerly zones are successi\-ely 

 encountered in regions whose altitude produces the requisite lov/er 

 temperature. By nearly all students of the Paleardic region, 

 on the other hand, it is supposed that the life areas of Europe and 

 Asia form large blocks, of no definite form and bearing no close 

 relation to isothermal boundaries. Thus, there is commonly recog- 

 nized a Eicropean subregion extending from the barren shores 

 of the Arctic Ocean to the luxuriant coast of the Mediterranean, and 

 embracing the great mountain mass of the Alps. This subregion is 

 by some writers contrasted with a Siberian subregion. considered 

 as of equal rank; by others united with it to form a Europasian 

 subregion. Other divisions of the PoJearctic region are the Man- 

 chiirian subregion, the Mediterranean sub-region, the Erentian sub- 

 region, etc. In all this there is the greatest diversity of opinion, 

 no two writers agreeing as to the number, names, or boundaries of 

 the subdivisions. In only one feature is there uniformity, in the 

 absence of that universal conception of zonal arrangement of the 

 life areas that characterizes the work of American writers on the 

 zoogeography of the Neardic region. In fact, one European author 

 has recently gone so far as to point out reasons why zonal distribu- 

 tion can not exist in certain portions of the Old World. 



But is it probable that such a radical difference in the distribution 

 of the life of America and Asia actually exists ? Does one law of 

 distribution hold good in one continent and a wholl}' different law in 

 the other, even though many of the animals and plants of the two 

 regions are by all writers admitted to be closely related ? Or 

 are the life zones of America represented by strictly homologous 

 areas in the Old World, dependent on the same underlying physio- 

 logical laws ? These questions are perhaps the most important nov/ 

 before the student of the general dispersion of life in the northern 

 hemisphere. 



Their answer was hinted at by several writers of half a century 

 ago, whose work is novv generally overlooked. Agassiz, in 1850, 

 v/ent so far as not only to recognize the zonal distribution of life in 

 the Alps and the legion lying to the north of them, but also to sug- 

 gest the correlation of the life zones which he had observed in Europe 

 with those of eastern North Ameri.ca. One of the present writers 



