ALLEN ON GEOGRAPHICAL DISTRIBUTION OF MAMMALS. 315 



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, and 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 by 

 itself."* 



In geographical zoology, as in the genetic relation of animals, we 

 find, as a rule, no strongly marked boundary -lines, and in the -life- 

 regions, especially those of lesser rank, the boundaries can be given 

 only approximately, owing to the intergradation of contiguous faunaB 

 and flora3, contingent upon the gradual modification of climatic condi- 

 tions; yet it is not hard to find boundary-lines that shall be, if not 

 sharply definable, at least easy of recognition. This at least proves to 

 be the case wherever the distribution of specific forms is thoroughly 

 known. The first objection, " that the several regions are not of equal 

 rank," forms to my mind no objection at all, since it matters little 

 whether they are equal or unequal if they correctly indicate the distri- 

 bution of life. 



The second objection Mr. Wallace has himself satisfactorily answered, 

 in discussing the question " Which class of animals is of most importance 

 in determining Zoological Regions." As Mr. Wallace here points out, and 

 as must become apparent to every careful investigator of this question, 

 the mammalia are pre-eminently of the greatest importance in deter- 

 mining zoological regions. To summarize Mr. Wallace's argument on 

 this point,t their dispersal is less dependent on fortuitous circumstances 

 than that of the representatives of other classes; from their high 

 organization they are less dependent upon " other groups of animals ", 

 and have so much power of adaptation that they are " able to exist in 

 one form or another over the whole globe", as is certainly not the case 

 with two of the lower classes of vertebrates, the reptilia and amphibia. 

 Their distribution and dispersal are dependent on the distribution of 

 the land-areas, and are modified by such physical conditions as mount- 

 ain barriers, areas of forest, and grassy or desert plateaus. Further- 

 more, their geological history, as well as their geographical range, is 

 better known than that of most other classes, and there is also a greater 

 unanimity of opinion respecting their natural affinities and the limita- 

 tion of families and genera in this class than in most others. " We 

 should therefore ", says Mr. Wallace (and I heartily agree with the re- 

 mark), " construct our typical or standard Zoological Eegions in the first 

 place, from a consideration of the distribution of mammalia, only bring- 

 ing to our aid the distribution of other groups to determine doubtful 

 points. Eegions so established will be most closely in accordance icith 

 * Geogr. Dist. Anim., vol. i, p. 53. t See Geogr. Distr. Anim., vol. i, pp. 56-58. 



