160 J. K. S. MOORE. 



thank Sir Harry Johnston for the very eflFective support he 

 lent the expedition^ and without which it would have been 

 impossible for me to attain the objects which I had in view. 



Excluding the polar regions proper, there exists in the 

 fresh waters of the different continents a type of fauna which 

 in the character of its constituents is essentially the same. 

 Certain forms are added and others are omitted as we pass 

 from the more temperate to the equatorial zones, but beneath 

 these changes there exists a substantial similarity, so easily 

 recognisable and so marked that geologists have not hesitated 

 to distinguish between fresh-water and marine fossiliferous 

 deposits wherever they may be found. On the other hand, 

 that there is a hard and fast demarcation between fresh- 

 water and marine faunas is not true, for there are many 

 instances of animals — for example, of prawns and crabs, which 

 in this country are purely oceanic — having made their way up 

 the rivers into inland and elevated fresh-water lakes. Further, 

 there are a number of animals that belong neither to salt nor 

 fresh water, but are inhabitants of the brackish regions which 

 lie between inland fresh waters and the sea. That the well- 

 established and more permanent fresh-water organisms of 

 the present day are descended from older phyla that were 

 once marine, is an accepted truth. This view is necessitated 

 by the theory of common descent, and it is supported, as in 

 the case of the ganoid fishes, by the similarity of numerous 

 liA'ing fresh-water organisms to older oceanic types. It is sig- 

 nificant, however, that with few or no exceptions, all the well- 

 established fresh-water organisms of to-day are not directly 

 referable to the earliest oceanic forms, but rather to those 

 which in their temporal distribution stand intermediate be- 

 tween then and now. It seems that the fresh-water molluscs 

 of the present day first make their definite appearance in 

 Tertiary times, for much doubt has recently been raised as to 

 the genuineness of the so-called carboniferous Unio, Ticho- 

 gonia, and Planorbis; these forms being now regarded 

 as more nearly related to Anthracosia, Avicula, and 



