8o FIELD AND FOREST. 



the continents were closely connected, directly or indirectly. The 

 former supposition is perhaps best applicable in some cases, as in the 

 dispersion of the sirenoids ; but the latter seems to be the more pro- 

 bable in the cases of most of the other forms. It is significant that 

 the similarity of the ichthyic faunas of South America and Australia is 

 exemplified chiefly on the western slope of the former, while the 

 forms common to it and Africa are characteristic especially of the 

 eastern slope. The moral from all these facts seems to be that the 

 birds, and the animals of analogous powers of extension, are the most 

 apt exponents of the present relations of land and water, while the 

 fishes, and animals of like restriction of locomotion, furnish the best 

 hints as to the ancient connections of the precursors of the existing 

 continents. 



Mr. Wallace asserts that the class of animals best adapted to deter- 

 mine zoological regions is the one which exhibits "by their existing 

 distribution the past changes and present physical condition of the 

 earth's surface" (Vol. i. p. 56), and that class, he thinks, is the mammals. 

 He maintains (vol. i. p. 57) that "we should therefore construct our 

 typical or standard zoological regions, in the first place, from a con- 

 sideration of mammalia, only bringing to our aid the distribution of 

 other groups to determine doubtful points." Mr. Wallace's argument 

 throughout is tantamount to the admission that the division into re- 

 gions is an arbitrary matter, and that there can only be a conventional 

 agreement as to those divisions. This is to a considerable extent true, 

 although it is probable that Mr. Wallace would object to this view 

 being the natural outcome of his argument. Here it may be premised 

 that in their indications the mammals are somewhat intermediate be- 

 tween the birds and fishes. And, by the way, we must wonder that 

 when Mr. Wallace considers the distribution of mammals as all-im- 

 portant, and that the "negative character of the absence of certain fami- 

 lies or genera is of equal importance" to the positive character of 

 their presence (vol. i. p. 54, ) he has refused to recognize the distinc- 

 tion of the Polynesian and New Zealand subregous of his Australian 

 region from the Australian and Austro-Malayan. * His reasons for so 

 doing (vol. i. p. 62) might be extended equally to the negation of one 

 at least of his admitted regions. 



* There are no indigeneous terrestial mammals in New Zealand or Polynesia, 

 while hey are richly developed in the Australian and Papuan regions. 



