310 THE HOLARCTIC REGION. [CHAP. 



analogue of the Sonoran practically equivalent to the Mediter- 

 ranean sub-region of European writers but says nothing about 

 an analogue of the Transition; from which I infer that he 

 would use the term Boreal really as practically equivalent to the 

 Holarctic, if the Sonoran and Mediterranean areas were subtracted. 

 In the New World the Boreal, exclusive of the Transitional, is 

 divided into an Arctic and a Coniferous Forest Boreal zone, the 

 latter being frequently spoken of as the Boreal " zone," in contra- 

 distinction to the Boreal circumpolar "region"; the Arctic zone 

 including the tract beyond the limit of trees. The distinction 

 between the Boreal and Transition areas is certainly not of 

 regional value ; and as the term Boreal is used in several senses, 

 it had better give place to the earlier Holarctic. 



As has been partially indicated in earlier chapters, and as will 

 be more fully noticed in the sequel, there is undoubtedly a marked 

 distinction between the mammals of North America as a whole 

 and those of Europe and northern Asia, but this has been con- 

 siderably exaggerated by including the Sonoran region in the old 

 Nearctic, and is overshadowed by the number of genera and 

 species common to the two areas and unknown elsewhere. Could 

 a Mediterranean region be satisfactorily denned, the homogeneity 

 of the mammalian Holarctic fauna would be still more apparent ; 

 but this, from the great mingling of northern and southern types 

 which has taken place in the Old World, is, I think, impracticable, 

 As has been already mentioned, it is probable that the western 

 and eastern halves of the Holarctic region have never had more 

 than a comparatively small area of communication by way of 

 Bering Strait, and, therefore, the further south we travel in the 

 two areas the more distinct do the faunas become; while only 

 such forms as are capable of withstanding a certain degree of cold 

 have ever been able to cross at all. It may be added that the 

 evidence for the unity of the Holarctic region is by no means 

 solely dependent upon the mammalian fauna, but is supported by 

 many other groups of animals. To take an instance from the 

 insects, I may quote from Mr W. F. Kirby l , who writes as follows : 

 " Had I been dealing with Lepidoptera only, I would certainly 



1 Joitrn. Linn. Soc. Zool. 1873, p. 432. 



