The most noteworthy features of the fauna are the preponderance of bats (16 species) and rats (13 species)--which together constitute nearly three-fourths of the total number of mammals known to occur in the islands--and the absence of practically all representatives of the ungulates, squirrels, carnivores, and flying lemurs, which are characteristic of the surrounding regions and abound on other islands at equal distance from the mainland. From the Malayan islands where these occur they differ in that "they are surrounded by water of relatively great depth, while the others lie within the 50-fathom line. This paucity of mammalian life cannot be regarded as due to an unfavourable environment, since all the natural conditions on both Andamans and Nicobars are perfectly suited to the support of a rich and varied fauna"; yet so great is it that it appears safe to assume that these, "contrary to the case with the shallow-water islands, were isolated at a time when the mammals now characteristic of the mainland did not exist there." In fact, we are almost driven to conclude that they never were at any time a portion of the continent, but were formerly only far nearer to it, far larger and far more compactly situated--a hypothesis that is further supported by an investigation of the birds appertaining to them. "As yet no species have been discovered whose origin may be referred to the remote period of a land connection: such mammals as are now known are evidently of very recent origin, as in scarcely an instance has their differentiation progressed further than in the case of members of the same genera found on islands lying in shallow water. The question at once arises, therefore, as to the means by which they have arrived where they now are. Flights from the mainland would readily account for the distribution of the bats; but the presence of the other mammals seems impossible to explain otherwise than through the agency of man. With the single exception of _Tupai nicobarica_,[219] all are types well known to be closely associated with man throughout the Malayan region. Moreover, the period of time necessary to the development of the peculiarities of the native Andamanese would undoubtedly be ample to allow the formation of any of the species known from either group of islands, since in a biologic sense it has been vastly longer to the smaller, more rapidly breeding, animals than to man. The introduction, intentional or otherwise, of a pig, a monkey, a palm-civet, two or three species of rats, a shrew, and perhaps also a tree-shrew, at about the time when the various islands were peopled by their present human inhabitants, would amply account for the existence of the present mammal fauna with its striking peculiarities."