Previous to entering into any details of the fauna of the Andamans and Nicobars, a glance at the depths of the surrounding ocean is interesting, and to a great extent explanatory of the peculiarities occurring in both groups: it is well known that the soundings of the adjacent seas clearly indicate the extent of time during which masses of land have been isolated, and the facts of this case seem to fully explain the variation and numerous peculiarities of the local fauna.
Preparis Island is situated at the tail of a 100-fathom (to be more particular, 50-fathom) bank projecting from the Arakan Yoma Peninsula. It is continental in its fauna, and possesses monkeys and squirrels.
Between it and the Cocos Islands is a depth of 150 fathoms.
The Andaman group, from Cocos to Little Andaman (except the South Sentinel, which is isolated), all stand on a 100-fathom bank (actually 50 fathoms).
All these are connected by a 200-fathom line with the Arakan Yoma Peninsula.
Narkondam and Barren Island both rise from a sea approaching 1000 fathoms in depth.
The Andamans and Nicobars are separated by a channel with depths of 600 fathoms.
Soundings about the Nicobars are at present very incomplete, but the Archipelago seems capable of division into two groups, each standing on a 100-fathom bank.
The northern of these consists of the compactly-situated central islands, and possibly Kar Nicobar, and is separated from the southern (Great and Little Nicobar and the adjacent islets, all perhaps surrounded by a 50-fathom line) by a channel with approximate depths of 200 fathoms.
The Nicobars stand at the termination of a 1000-fathom bank, projecting from the Arakan Yoma Peninsula, and from thence also curving east and south towards Sumatra, thus enclosing a long tongue of deep sea, over 1000 fathoms deep, that is connected with the Indian Ocean by the channel separating them from Sumatra.