- DidriJ)ult(»i of Life in Oircnntpolar Zones. .")." 



(Ic^Tee of Iicnt in (liscomuctcd land areas — a manifest inipossi- 

 Mlitv — l)ut that well niai'kcM] /ones of animal and plant life are 

 eneountered in all [(arts of the earth in passing from the poles 

 to the tropics : that tliev owe their existence to constant differ- 

 ences of tenii>eratuve. and that in continuous land areas each 

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

 ocean in those of continental magnitude], following the windings 

 of the helts of equal temperature during the })eriod of reproduc- 

 tive activity. 



\\'allace speaks thus of this law as formulated ])y Allen : " The 

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

 fioi) of life ill rirciiiiipolar zones,^ 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 ^ve 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 

 (/enera and fo III ilie.'^ to certain areas."' (Geog. Dist. of Animals, 

 vol. 1, 1876, p. (M). Mr. Allen has already pointed out tlie weak- 

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

 vol. TV, No. 2, May, 1878, 32B), and I would hke to add a word 

 respecting the extraordinary^ statement that circumpolar 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 

 Xortli Ameriea is distinguished from the Boreal by the posses- 

 sion of 7 families and o4 genera of mammals alone,* and the 

 North American Tropical from the Sonoran by 10 families and 

 upwards of 50 genera ; wdiile the American Boreal differs from 

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

 only 8 genera. 



* These genera are: Diddphi% Dicoti/les, Cariacus, Antilocapra, C'!pinii)!/i<, 

 Beithrodonfniiiiis, Oni/rliomiiK, Oriizonn/s, Si{/mo(Ion, Neotoma, Geoinys, TIioiiio- 

 mys, Dijioiloiiii/s, Perudlpiix, Microdipodops, Pewgnathus, Heteromys, Fdls, 

 Urocyon, Procyon, Bassariscus, Taxidm, Conepatus, Mepliitis, Spilogalc, Xo- 

 tiosore,v, Scalojjs, ('onjnorlihins, Eudt'rma, Antrozous, Nycticfjin^, Molomof, 

 NycfhwmnK, and Otojjfn-us. Five of these genera have each a 8]iecies 

 reaching a sliort distance into the sontliern edge of the Boreal Region, 

 namely, ('ar(<inif<, Xiofaiua, lulis, I'rocijnri^ and Mi'philix. 



