10 



others as unaccountable as in the three instances first men- 

 tioned. 



For example, the Diamond Snake is never found in the 

 country lying west of the Upper Derwent and Lake St. Clair, 

 though other species are numerous. The large and beautiful 

 Mountain Duck or Shieldrake (Casarca tadornoides) breeds in 

 large numbers on one small salt water swamp near Swanport, 

 and is never seen except in pairs as casual visitors in other 

 swamps to our perceptions the very counterparts of that at 

 Swanport. The little mountain rivulets running through 

 New Town, G-lenorchy, and Hobart Town, possess two species 

 of small fish from the sea upwards for two or three miles, 

 which species then gradually give place to a closely allied 

 third species, not found in the lower streams, no difference in 

 the food, temperature, or other physical aspects of the water 

 being apparent to our senses. 



The large fresh water fish known to northern colonists as 

 the " Black Fish," and the great river Crayfish abound in 

 streams flowing to the northern and western coasts, and are 

 wanting in those flowing to the eastern and southern. 

 Although in the case of the Gordon, flowing west, and the 

 Derwent, flowing east and south, some of the sources of each 

 river approach very near to the same spot, and the crustaceans 

 might actually walk through swamps from one to the other. 

 The large Freshwater Mussel is present in the Macquarie, 

 and absent in the Jordan, though the rivers are of much the 

 same character. And the graceful Forrester rat (Rypsi- 

 prymnus apicaUs), plentiful all over the Bagdad tier, is never 

 seen on the hills round Hobart Town, possessing the same 

 climate and vegetation. So I might go on drawing instances 

 innumerable from many sections of the animal kingdom, but 

 those adduced will be sufficient for my present purpose. 



With reference to the Magpie, Miner, and Rosella, Mr. 

 Calder wrote me some time ago that he had been in the habit 

 of frequently walking in the domain for 35 years past, and 

 had never seen any of those birds there till the two preceding 

 years, and concluded by asking what could cause their absence 

 formerly or their presence then ? 



My own belief is, that the well-being of living creatures in 

 their wild state often hangs upon a very slender thread, and 

 that in such case but a slight alteration in the balance already 

 established by nature may produce effects infinitely greater 

 than any one would be, at first sight, prepared to admit. Is 

 it not certain that the gradual destruction of any one creature, 

 and the consequent increase of the food upon which that 

 creature preyed, will make room for neighboring animals 

 preying upon the same food ? 



