Aug.. 1893. THE DISTRIBUTION OF BIRDS. 101 



of any other portion of the globe, with the exception, perhaps, of our 

 own islands. 



While, therefore, many of my conclusions with regard to the 

 extent of the provinces of the Old World must be purely conjectural 

 from want of definite knowledge, there are certain sub-regions which 

 may be suggested, and their boundaries defined, with some confidence. 



Some of Mr. Allen's conclusions ("Auk," 1893, pp. 97-150) with 

 regard to the main divisions of the Old World are the same as those of 

 Dr. Reichenow, and I think that they are, in both instances, too sweep- 

 ing ; but the recognition and definition of an Arctic Zone or " Realm," 

 as Mr. Allen calls it, is a fact which must henceforward be admitted 

 by all ornithologists. Its southern boundary is probably along the 

 isothermal line of 50 (mean summer temperature), and may be 

 coincident with the northern limit of conifers. 



Mr. Allen's " North Temperate Realm " includes both hemi- 

 spheres to the northern border of the palm belt. Its southern line 

 corresponds with the isotherm of 77 mean annual temperature, but 

 this constitutes a very roughly defined southern border for the zoo- 

 logical areas of the Old World. 



The southern border of palm-growth follows the southern 

 isothermal line of 77 (mean annual temperature) but by no means 

 defines natural borders for Mr. Allen's Indo- African Realm or Dr. 

 Reichenow's ^Ethiopian and Malayan Regions, and it is better, 

 as it seems to me, to stick to our old terms at present, instead of 

 defining the limits of the Old World regions and sub-regions, with 

 their ill-tabulated data, by analogy with the New World sub- 

 regions, the boundaries of which are so much better known. It is 

 for this reason also that I have not adopted Mr. Allen's nomenclature 

 in its entirety. I may in due time be brought to speak of " Realms," 

 but the same conservatism which prevents my adopting the trinomial 

 nomenclature of the American zoologists of the present day, will 

 prevent my discarding some of the old-fashioned, and, to me, 

 expressive zoo-geographical terms. I cannot understand why the 

 word " Nearctic " should be discarded. It was given by Dr. Sclater 

 not in the sense of " arctic " but " northern " region of the New 

 World, and is, in my opinion, apart from the priority which 

 commands respect for its retention, a most simple and expressive 

 term. My American colleagues will understand that if I have not 

 carried their system of nomenclature into the zoo-geographical regions 

 of the Old World, it is not from any want of respect to their work, 

 for I heartily agree with their conclusions as regards North America ; 

 but a little time must elapse before we can confidently apply the 

 same reasoning to the avifauna of the Old World. When Dr. 

 Pleske and Professor Menzbier have finished their works on the birds 

 of the Russian Empire, so that the confusion caused by the "lumping " 

 tendencies of the Middendorffian school, so justly deplored by Dr. 

 Stejneger, shall have been dispelled ; when France, and Spain, and 



