Relative Values of Humidity and Temperature. 47 



EFFECTS OK HUMIDITY CONTRASTED WITH EFFECTS OF 

 TEMPERATURE. 



With few exceptions, the 'Boreal zones, owing to their low 

 temperatures, precipitate sufficient moisture to support arboreal 

 vegetation and do not possess arid arenas. The Transition and 

 Sonoran zones on the other hand naturally fall into two impor- 

 tant subdivisions, arid and humid, as indicated in defining 

 their courses. As a rule the former consist of treeless plains, 

 deserts, and barren mountains, while the latter are bountifully 

 clothed with forests. Most of the humbler forms of vegetation 

 are different in the two subdivisions, and differences exist also 

 among the mammals, birds, and reptiles ; but the great majority 

 of these dissimilarities are not of the same kind as those that 

 distinguish one zone from another. Most of them are specific 

 not generic and the number of distinctive groups of high order 

 is very much less. This may be made clear by selecting the 

 distinctive elements of the arid Sonoran (which has the largest 

 number of peculiar forms) in comparison with those of the humid 

 Sonoran (or Austroriparian) and contrasting them numerically 

 with the distinctive elements of the Sonoran as a whole compared 

 with those of the Boreal as a whole.* Among non-pelagic mam- 

 mals, the arid Sonoran has one family (Antilocapridze) and only 

 ten genera f not known to inhabit the humid Sonoran or Aus- 

 troripariaii ; and the latter has but one family (Didelphidae) and 

 four genera (Didelphis, Oryzomys, Scalops, and Nycticejus) not 

 found in the arid Sonoran (and the family and one of the genera 

 are intrusions from the Tropical region), while 13 families and 

 27 genera are common to both arid and humid subdivisions. % 



Among birds, the arid Sonoran has no family and only 24 

 genera not inhabiting the humid Sonoran, and the latter has 

 no family and but 7 genera not found in the arid, while 12 fam- 

 ilies and 31 genera are common to the two divisions. 



Contrasting the Sonoran as a whole with the Boreal as a 

 whole, it appears that there are no less than 8 families and 41 



*The intrusive Tropical genera are here treated as Sonoran. 



f These genera are : Anttiocapva, Cynomys, Onychpmys, Thomomys, Dlpo- 



x, J'wodipus, Microdipodops, Perognathus, Basmriscus, and Antrozous. 

 I The newly discovered genus of (Jhiroptera, Euderma, is here omitted 

 because only a single specimen is known and it cannot yet be satisfac- 

 torily assigned to its proper fauna! position. 



