48 Merriam — Geographic Distribution of Life. 



genera of nuiiiinials iind 10 Ikmilies and about 100 genera of 

 l)irds 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 l)irds together, the arid Sonoran has one peculiar famil}'- 

 and only 34 distinctive genera, and the humid Sonoran one 

 family and 11 genera (of which the family \_Di(MphldR'] and 

 several of the genera are clearly intrusions from the Tropical 

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

 18 distinctive families and 141 distinctive genera, and the Boreal 

 has 9 distinctive families and 70 distinctive genera. 



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

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

 vi(hr, Murirhe, SciiiricLr, Lcporiibf, MustelidcV, Canichv, FelicLr, and 

 So7-ici(br. The common genera are : Sitomys, Sciurus, Sciuropterus, 

 SpennopJiilus, Lepvs. Liitra, Canis, and Lynx. Several others in- 

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

 regions as a Avhole. 



^^4th the possilile exception of the gray wolf, not a single 

 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 x\rctic America. The ermine is another 

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

 true that the weasel inhalnting 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 Tardidie or thrushes inhabit 

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

 the Paridiie or titmice inhabit North and South America, Eurasia. 

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

 pers inhal)it North and South America, Eurasia, India, and the 

 Austro-Malayan region ; the Troylodytidpe or wrens inhabit North 

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

 Malayan region; the (hrrid;r or crows, magi)ies and jays, are 

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



