THE TANGANYIKA PROBLEM. 17 



characterising the primary fresh-water series, we find that 

 these types, such as Vivipara, Planorbis, Limited, and 

 Melania among the molluscs, fresh-water prawns and 

 crabs among the Crustacea, and the immediate forerunners 

 of the now widely dispersed groups of fresh-water fishes, 

 such as the ganoids, arose as such about the same time. 

 The emergence of the primary fresh-water series being as 

 a matter of fact synchronous with a strange phenomenon 

 already well recognised by geologists ; it occurs just at 

 the time when a curious break is manifest in the forms 

 characteristic of the secondary and tertiary deposits, and 

 from this we might infer, or at least think it probable, that 

 they owe their differentiation to the same cause which 

 produced during this period an extraordinary multiplication of 

 new types and the extinction of old ones. From the manner 

 in which the primary fresh-water series appears in the 

 geological record, it in fact seems to be suggested that 

 there came into play some cause which was efficient to kill 

 out a disproportionately large number of ancient marine 

 types, and at the same time, both to produce a dispropor- 

 tionate array of new marine forms, and to dissociate from 

 these the representatives of the primary fresh-water series 

 as universal inhabitants of the waters of the land ; that is to 

 say, the facts of morphology and the facts of palaeontology, 

 when taken together, seem to suggest that there has 

 occurred something like a change in the character of the 

 sea itself, which has affected the animals contained in it in 

 such a manner that a large number of its old forms were 

 definitely killed out, while others were driven into the 

 fresh waters of the globe, and at the same time a very 

 large number of entirely new marine types was produced. 

 In attempting to form any clear conception of the nature 

 and the origin of the curiously similar fresh-water faunas 



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