6 BIOLOGICAL SOCIETY OF WASHINGTON. 



animals. As to the first objection, it will be found impossible to form 

 any three or more regions each of which differs from the rest in an 

 equal degree or in the same manner. One will surpass all others 

 in the possession of peculiar families; another will have many 

 characteristic genera ; while a third will be mainly distinguished by 

 negative characters. There will also be found many intermediate 

 districts, which possess some of the characteristics of two well" 

 marked regions, with a few special features of their own, or perhaps 

 with none ; and it will be a difficult question to decide in all cases 

 which region should possess the doubtful territory, or whether it 

 should be formed into a primary region itself. ' ' * 



As to the question " Which class of animals is of most importance 

 in determining Zoological Regions," Mr. Wallace thinks that we 

 should " construct our typical or standard Zoological Regions in the 

 first place, from a consideration of the distribution of mammalia, 

 only bringing to our aid the distribution of other groups to determine 

 doubtful points. Regions so established will be most closely in ac 

 cordance with those long-enduring features of physical geography, on 

 which the distribution of all forms of life fundamentally depends ; and 

 all discrepancies in the distribution of other classes of animals must 

 be capable of being explained, either by their exceptional means 

 of dispersion or by special conditions affecting their perpetuation 

 and increase in each locality." "If these considerations are well 

 founded," he continues, "the objections of those who study in 

 sects or molluscs, for example, that our regions are not true for their 

 departments of nature, cannot be maintained. For they will find, 

 that a careful consideration of the exceptional means of dispersal 

 and conditions of existence of each group, will explain most of the 

 divergences from the normal distribution of higher animals, "f 



Mr. Allen recalls that he had in 1871 "claimed, in accordance 

 with the views of Humboldt, Wagner, Dana, Agassiz, DeCandolle, 

 and others, that life is distributed in circumpolar zones which con- 



* Wallace, Geog. Dist. Anim., vol. i. p. 53. 

 f Wallace, op. cit., vol. i, p. 57. 



