268 PROCEEDINGS OF THE ACADEMY OF [1883. 



being such by reason of their being held in common by the two 

 regions ; and (3) that the Nearctic and Palsearctic faunas taken 

 collectively are more clearly defined from any or all of the other 

 faunas than either the Nearctic or Palaearctic taken individually, 

 " In reference to these points, Mr. Wallace, while not denying 

 the facts, remarks : ' The best division of the earth into zoo- 

 logical regions is a question not to be settled by looking at it 

 from one point of view alone ; and Prof. Heilprin entirely omits 

 two considerations — peculiarity due to the absence of widespread 

 groups, and geographical individuality.' Numerous families and 

 genera from the classes of mammals and birds are then cited as 

 being entirely wanting in the western hemisphere, and which — 

 in many cases almost sutflcient to ' characterize the Old World 

 as compared with the New ' — ' must surely be allowed to have 

 great weight in determining this question.' No one can deny 

 that the absence from a given region of certain widespread 

 groups of animals is a factor of very considerable importance in 

 determining the zoological relationship of that region, and one 

 that is not likely to be overlooked by any fair-minded investi- 

 gator of the subject. But the value of this negative character 

 afforded by the absence of certain animal groups as distinguish- 

 ing a given fauna, is in great measure proportional to the extent 

 of the positive character — that furnished by the presence of 

 peculiar groups - and indeed may be said to be entirely depen- 

 dent on it. No region can be said to be satisfactorily distin- 

 guished from another without its possessing both positive and 

 negative distinguishing characters, Mr. AVallace has in his 

 several publications laid considerable stress ui)on the negative 

 features of the Nearctic fauna as separating it from tlie Palaj- 

 ai'ctic or from any other, but he has not, it appears to me, suffi- 

 cientl}' emphasized the great lack, lohen compared to other 

 faunas^ of the positive element, the consideration of which is the 

 point aimed at in the first portion of my paper, and which has 

 led to the conclusions already stated — that only by uniting the 

 Nearctic and Pala?arctic regions do we produce a collective 

 fauna which is broadly distinguished by both positive and nega- 

 tive characters from that of any other region. If, as Mr. 

 Wallace seems to argue, the absence from North America of 

 the ' families of hedgehogs, swine and dormice, and of the 

 genera Mehs, Eqiius, Boh, Gazella, 3Ias, Cricelus, Merionen, 



