It would be impossible, within the limited space at our disposal, to discuss in detail the different subjects dealt with in the twenty-three chapters into which the work is divided, and we must content ourselves with presenting to our readers a faint outline of the general argument.
Mr. Wallace is of opinion that the existing continents and deep oceans are of great geological antiquity; that during the eocene period the bulk of the land was on the northern side of the Equator; and that in that hemisphere the struggle for existence was more severe than in the southern hemisphere. Consequently the highest and most specialized forms of animal life are to be found north of the Equator. In supporting his views, with much sound reasoning, Mr. Wallace has adopted the six zoological regions first proposed by Mr. Sclater in 1857, and these are now generally accepted as natural divisions of the earth's surface. With the exception that Mr. Sclater's Indian Region is altered by Mr. Wallace, in name only, to the Oriental Region, no other important change is suggested. The six regions accordingly stand as the Palæarctic, the Ethiopian, the Oriental, the Australian, the Nearctic, and the Neotropical; each being divided into four subregions.
The only subregion in which no mammals exist, except Bats, is New Zealand; and Mr. Wallace explains this fact by supposing that New Zealand has not been connected with any other part of the earth's surface since the creation of the Mammalia. The great region of Australia is almost destitute of placental mammals, showing, according to our author, a very ancient isolation of that part of the world.
The various cases of the existence of isolated forms in South America and South Africa are also explained by the supposition that these regions, separated in remote geological times, were large islands, and the struggle for existence there not having been so severe as in the northern hemisphere, low forms of Mammalia, such as the Edentata, Caviidæ, and others have survived to the present day. We are very much disposed to accept this reasoning, and it well explains the existence of such low forms of birds as Apteryx in New Zealand, the Tinamidæ, Dicholophus, and others in South America, Rhinochætus in New Caledonia, and such an aberrant form of the Accipitres as Serpentarius Africa.
In the opinion of Mr. Wallace, the present state of the globe is one of zoological depauperization, caused by the glacial period.
