THE TANGANYIKA PROBLEM. 
i7 
characterising the primary fresh-water series, we find that 
these types, such as Vivipara, Planorbis , Limnea , 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 
2 
