Distribution of Life in Circumpolar Zones. 5") 



decree <>f heat in disconnected land areas a manifest impossi- 

 bility hut that well marked zones of animal and plant life are 

 encountered in all parts of the earth in passing from the poles 

 to the tropics; that they owe their existence to constant differ- 

 ences of temperature, and that in continuous land areas each 

 zone may be traced completely across such areas [from ocean to 

 ocean in those of continental magnitude], following the windings 

 of the belts of equal temperature during the period of reproduc- 

 tive activity. 



Wallace speaks thus of this law as formulated by Allen : " The 

 author [J. A. Allen] continually refers to the ' law of the distribu- 

 tion of life in circtnnpolar zones," 1 as if it were one generally accepted 

 and that admits of -no dispute. But this supposed ' law ' only 

 applies to the smallest detail of distribution to the range and 

 increasing or decreasing numbers of species as we pass from north 

 to south, or the reverse; while it has little bearing on the great 

 features of zoological geography the limitations of groups of 

 (jcucra and families to certain areas/' (Geog. Dist. of Animals, 

 vol. 1, 1876, p. 67). Mr. Allen has already pointed out the weak- 

 ness of this criticism (Bull. U. S. Geol. and Geog. Survey Terr., 

 vol. IV r , No. 2, May, 1878, 326), and I would like to add a word 

 respecting the extraordinary statement that circum polar distri- 

 bution affects species only, having " little bearing " on the " limi- 

 tations of groups of genera and families." In refutation of this 

 fallacy it is hardly necessary to do more than call attention to 

 the circumstance that the transcontinental Sonoran region of 

 North America is distinguished from the Boreal by the posses- 

 sion of 7 families and 34 genera of mammals alone,* and the 

 North American Tropical from the Sonoran by 10 families and 

 upwards of 50 genera ; while the American Boreal differs from 

 the Eurasian Boreal' by the possession of but a single family and 

 only 8 genera. 



* These genera are : Didelphw, Dicotyles, Cariacus, Antilocapra, Cynomys, 

 Reithrodontomys, Onychomys, Oryzomys, Sigmodon, Neotoma, Geomys, Thomo- 

 mys, Dipodomys, Perodipus, Microdipodops, Perognathus, Heteromys, Felis, 

 Vrocyon, Procyon, Bassariscus, Tax-idea, Conepatus, Mephitis, Spilogale, No- 

 tumore.c, ficalops, Corynorhinus, Euderma, Antrozous, Nycticejus, Molossus, 

 Nyctinomus, and Otopterm. Five of these genera have each a species 

 reaching a short distance into the southern edge of the Boreal Region, 

 namely, (.'ariiim..*, X<'<>t<n<i, AW/'.s-, Procyon, and Mephitis. 



