of Bronn's _Thier-Reich_, and thus the opinions of each were in many cases mutually affected. Dr. Gadow, on the completion of his undertaking, propounded a scheme of classification, which is followed, with some slight modifications, in the present volume (see foregoing table)–it being, of course, understood that a linear arrangement is, {15}strictly speaking, impossible, since any group may have a decided affinity to more than two others. This Classification, beginning (as Birds themselves must have begun) with the lower forms, takes us, except in the _Oscines_, as far as the Families, which in most cases are fairly distinguishable, though of very variable value. Coming to Genera, and still more to Species, the opinions of authorities often differ so widely, that at present an attempt to reconcile them is hopeless. It cannot be denied that Genera and Species are merely "convenient bundles," and that divisions of either, if carried too far, defeat the object for which Classification is intended. Genera are only more distinct from Species, and Species from Races, because the intervening links have disappeared; and, if we could have before us the complete series which, according to the doctrine of Evolution, has at some time existed, neither Genus nor Species would be capable of definition, any more than are Races in many cases; while the same remark will apply to the larger groups. From these Races or Geographical variations we may not unnaturally turn to GEOGRAPHICAL DISTRIBUTION. It will always be credited to Ornithology that the interesting study of the Geographical Distribution of Animals was first placed on a scientific basis as a result of the study of Birds. This was effected by Mr. Sclater, whose division of the Globe into Six "Regions"–the _Palaearctic_, _Ethiopian_, _Indian_, and _Australian_, forming one group–the "Old World" (_Palaeogaea_); and the _Nearctic_ and _Neotropical_, forming a second–the "New World" (_Neogaea_); was announced in 1858 (_J. Linn. Soc._ ii. pp. 130-145). His scheme, being solely grounded on Ornithological considerations, was accepted with scarcely any modification by Mr. Wallace in his great work (_Geograph. Distrib. of Animals_, 1876), and by the majority of zoologists, though some demurred, and among them Huxley, who, in especial reference to Birds, shewed (_Proc. Zool. Soc._ 1868, pp. 313-319) that there was more reason to divide the earth's surface latitudinally than longitudinally, and that Four Regions were better than Six–these four being (1) _Arctogaea_, comprising Mr. Sclater's Indian, Ethiopian, Palaearctic, and Nearctic; (2) Austro-Columbia, corresponding with the Neotropical; (3) Australasia; and (4) New Zealand–the last three being combined as _Notogaea_. In 1882 Prof. Heilprin proposed to unite Mr. Sclater's Palaearctic and Nearctic under {16}the name of Triarctic; but in the next year (_Nature_, xxvii. p. 606) adopted for that union Prof. Newton's earlier term Holarctic.