48 Merriam Geographic Distribution of Life. 



genera of mammals and 10 families and about 100 genera of 

 birds distinctive of the Sonoran, and 6 families and 30 genera 

 of mammals and 3 families and about 40 genera of birds dis- 

 tinctive of the Boreal zone. In other words, taking mammals 

 and birds together, the arid Sonoran has one peculiar family 

 and only 34 distinctive genera, and the humid Sonoran one 

 family and 11 genera (of which the family [Didelphidni] and 

 several of the genera are clearly intrusions from the Tropical 

 region), while the Sonoran as contrasted with the Boreal has 

 18 distinctive families^nd 141 distinctive genera, and the Boreal 

 has 9 distinctive families and 70 distinctive genera. 



Only 8 families and 8 genera of mammals are common to the 

 Boreal and Sonoran Regions. The common families are : Cer- 

 vidse, Muridse, Sciuridae, Leporidse, Mustelidde, Canidfe, Felidie, and 

 Soricidae. The common genera are : Sitomys, Sciurus, SciuropteruSj 

 Sperm-ophilus, ' Lepus, Lutra, Canis, and Lyn.r. Several others in- 

 habit limited parts of both regions, but are not common to these 

 regions as a whole. 



With the possible exception of the gray wolf, not a single 1 

 species of mammal ranges throughout the Sonoran and Boreal 

 Zones, though a number are common to the Upper Sonoran 

 and Lower Boreal (Canadian) ; and in the case of the wolf it is 

 almost certain that comparison of specimens will show the ani- 

 mal of the southern United States and Mexico to be perfectly 

 distinct from that of Arctic America. The ermine is another 

 species of phenomenal though less extensive range, if it is really 

 true that the weasel inhabiting the shores and islands of the 

 Polar Sea is specifically identical with that found in the more 

 elevated parts of the Southern States an assumption I cannot 

 for a moment entertain. 



In the case of land birds, 18 genera are common to the Boreal 

 and Sonoran Regions. The number of common families is rela- 

 tively large as would be expected from the wide dispersal of most 

 families of birds. For instance, the Tardidse, or thrushes inhabit 

 North and South America, Eurasia, Africa, India, and Australia ; 

 the ParidsB or titmice inhabit North and South America, Eurasia, 

 Africa, India, Australia, and New Zealand ; the Oindidx or dip- 

 pers inhabit North and South America, Eurasia. India, and the 

 Austro-Malayan region ; the Troglodytidas or wrens inhabit North 

 and South America. Eurasia, India, Africa, and the Austro- 

 Malayan region; the ('orndu- or crows, nnurpii's and jays, arc 

 found in every part of the world, and so on. 



