( 465 ) 



is also true for a specimen which I described from Ambon and still 

 more so for a specimen that I caught near Balangnipa in the lower 

 part of the Tangka, close to its mouth in the gulf of Boni. The 

 water was here already brackish and ran slowly. Rhiacichtys has 

 therefore a rery wide distribution, it does not fear brackish water, 

 and its presence in New Guinea loses therewith much of its importance. 



2. The second group, the characteristic element, is Australian. 

 This last group requires further explanation as to its origin. In the 

 present state of things, now that New Guinea is separated from 

 Australia by Torres Straits, these offer a barrier impassable to those 

 lishes which I called characteristic. Some species of Rhombatractus 

 and Melanotaenia may it is true, descend to the mouth of the river 

 and be able to endure even slightly brackish water, but none of the 

 24 recorded species is known from the sea. The barrier can therefore 

 not be bridged by the group of islands in the Torres Straits. They 

 are too poorly supplied with fresh-water and far too strictly coral 

 islands, even when we leave out of consideration the fact that they 

 are separated from each other, from New Guinea and from Australia 

 by broad tracts of sea with a high salt percentage and strong tidal 

 currents. The simultaneous presence of these characteristic forms in 

 New Guinea and in Australia cannot be explained otherwise than by 

 the existence of a more solid and extensive connection in former 

 ages. This connection must have been so far back in the past that, 

 to take an instance, the representatives of the abovenamed Melano- 

 taeniideae had time to separate themselves specifically. And this 

 actually happened; for among the 12 species of Melanotaeniidae 

 already known from New Guinea and among the 12 species described 

 from tropical or sub-tropical Australia not one is common to the two 

 regions, although the differences between some species are very 

 small. On the other hand therefore it cannot have been so very 

 long ago from a geological point of view that this connection between 

 Australia and New Guinea existed. How long a time may have 

 elapsed since that period is at present a matter of hypothesis. But 

 if zoo-geographical and more particularly ichthyological experience 

 may venture an opinion, 1 should seek the period of this connection 

 not earlier than in the pliocene, and the breaking up of it in the 

 pleistocene. Other zoological observations may perhaps be in favour 

 of this supposition. 



It will be a long time yet before the last word is spoken on this 

 question. We may express the hope that the new expedition to Dutch 

 Southern New Guinea under the guidance of Dr. H. A. Lorentz, which 

 intends to investigate especially its big rivers, will bring us further light. 



