Lesser slibdivi;imts lie delerinilleil inde pendently, if at all, for each group under study. N•r have even the largest zo.:;geographieal divi ion• hard and fast boundaries. They overlap and blend, forming transitional zones or debat able grounds. The stretch of mountainous des erts from the western Sahara to Manchuria is such a zone. where representatives of northern and southern faunas mingle. and where dwell species not known elsewhere in either. The so-called 'Sonoran' subregion (southwest United States to the highlands of Central America) is another borderland of debatable validity. The distinction between the Malayan and Australian subregions insisted upon by Wallace disappears entirely in respect to frogs and toads. In truth, the lesser subdivisions that have been nu merously and exactly marked off by specialists have no real existence. "They depend," to use Gadow's words (1902), "upon the class. or even order, of animals which we may happen to study. The faunistio distribution of the Urodela is not that of the Altura. and both follow separate lines of dispersal. different from those of the various orders of reptiles, birds, and mammals. This
must be so. There is no doubt that the distribu tion of land and water was totally different in the Coal Age from what it is now. The face of the globe at the -Jurassic Age can scarcely be com pared with the aspect which the world had as sumed in the Miocene Period. This leads to an other consideration often neglected. We know that the various orders, families, etc.. of animals have appeared successively on the stage. A group which arose in the Coal Age followed lines of dispersal different from one which was not evolved until Jurassic times, the post-Cre taceous creatures could not avail themselves of what assisted their an•e•tors. and vice versa.
. . Speaking generally. the older a group the more likely it is to be widely distributed. If it appears scattered, this may be due to extinc tion in intermediate countries, or to submergence of former land connexions. . . . It is the morphologist who is ultimately responsible for the establishment of fattnivtie regions. not the systematist, least of all he who accepts an elab orate classification. and then mechanically. mathe matically, by lists of genera and species, maps out the world."