The geographical distribution of animals, with a study of the relations ...

398 ZOOLOGICAL GEOGRAPHY. [part hi.The Australian region, then, possesses examples of18 familiesof Mammalia, 8 of which are peculiar ; 71 of Birds, 16 beingpeculiar; 31 of Eeptiles, 4 being peculiar ;11 of Amphibia, with1 peculiar; and 11 of Fresh-water fish, with 1 peculiar. In all,142 families of Vertebrates, 30 of which are almost or quiteconfined to it, or between one-fourth and one-fifth of the wholenumber.The genera of Mammalia, occurring within the limits of thisregion are 70, of which 45 are almost, or quite, confined to it.Of Land-Birds there are 296 genera, 196 of which are equallylimited. The proportion is in both cases very nearly fiveeighths.This shows a considerable deficiency both in families of Vertebratesand genera of Mammalia, as compared with the Orientaland Ethiopian regions ; while in genera of Birds it is a littlesuperior to the latter in total numbers, and considerably so inthe proportion of peculiar types.Supposed Land Connection between Australia and South America.We may now consider how far the different classes and ordersof vertebrates afford indications that during past ages therehas been some closer connection between Australia and SouthAmerica than that which now exists.Among Mammalia we have the remarkable fact of a groupof marsupials inhabiting South America, and extending eveninto the temperate regions of North America, while they arefound in no other part of the globe beyond the limits of theAustralian region ; and this has often been held to be evidenceof a former connection between the two countries. A preliminaryobjection to this view is, that the opossums seem to berather a tropical group, only one species reaching as far as 42°south latitude on the west coast of South America ; but whateverevidence we have which seems to require a former unionof these countries shows that it took place, if at all, towards theircold southern limits, the tropical faunas on the whole showingno similarity.This is not a very strong objection, since climatesmay have changed in the south to as great an extent as we