TRANSACTIONS OF SECTION D. 423 
they were easily carried into the T'anganyika—probably at no very remote time— 
and maintained themselves to the present day. I understand that the Medusa 
reported from Bammaku, Upper Niger, in 1895, but still undescribed, has been 
rediscovered by Budgett, and is now being studied. Should it prove to be related 
to the Tanganyika species, it would also have to be regarded asa relic of the 
same Eocene sea, and it would add further support to the very simple explanation 
which I have ventured to offer of a case which seemed so tremendously puzzling 
in our previous state of ignorance of the geological conditions of Africa between 
the equator and the tropic of cancer. 
As explained by Professor Cornet, Tanganyika has been until very recent 
times without an outlet. The Lukuga, which drains into the Congo, was only 
formed after Lake Kivu became, owing to volcanic eruption, a tributary of the 
Tanganyika through the Rusisi River. The greater or less salinity of the water 
of a lake without an outlet is a matter of course, and therefore Tanganyika was 
for a long time a salt lake. Its water is still, Mr. Moore says, somewhat salt. 
No wonder that the Cichlids, which elsewhere in Africa show no aversion to such 
conditions, and which somehow or other contrive to settle into isolated waters, 
should have been among the first inhabitants of the lake, where, without having 
to face competition with other types of fishes, they throve and became 
differentiated into a multitude of genera. When the hydrographical conditions 
changed and the water gradually lost its salinity, first on the surface and later at 
greater depths, an influx of other forms of fish-life (Polypterus, Characinids, 
Cyprinids, Silurids, &c.) penetrated into the lake, some from the Nile system 
through the Rusisi, others from the Congo up the Lukuga. This explains well 
enough the character of the Tanganyika fish-fauna. The Cichlids, the oldest 
inhabitants of the lake, nearly all belong to endemic species, many of which consti- 
tute genera represented nowhere else; whilst the fishes of other families, later immi- 
grants, all belong to widely distributed genera, and several of them even to species 
also found either in the Nile or in the Congo, or in both these river-systems. 
The other theory is that the Cichlids have originated as fresh-water fishes in 
Eocene times in America and have crossed the Atlantic by a bridge which then 
connected South America with Africa. This is the explanation given by Dr. 
Pellegrin. He admits that we have no indication of any near allies of these 
fishes before the Middle Eocene (Green-River beds of North America), and, 
basing his statement on the last edition of Professor de Lapparent’s ‘ Traité de 
Géologie’ (1900), he says it seems to be beyond doubt that during the Lutetian 
period, which immediately followed that at which the earliest Cichlids were 
known to live in the fresh waters of America, a vast continent extended between 
South America and Africa. Should this have really been the case, the question 
of the distribution of the Cichlids could be regarded as settled. But 1 cannot 
satisfy myself that there is any geological evidence to support this view. That a 
shallow sea with a chain of islands connected the West Indies with the Mediter- 
ranean in Middle Eocene times is generally believed, and this theory is supported 
by serious arguments; but a series of islands would hardly be suflicient for the 
passage of fresh-water forms, such as we have reason to think the Cichlids have 
always been. In his maps of distribution Dr. Ortmann does not support Dr. 
Pellegrin’s assumption. Professor de Lapparent has kindly informed me that his 
map of the Lutetian period requires very considerable modifications owing to the 
discoveries in the Soudan, in Senegambia, and in Cameroon, to which allusion has 
been made above; in fact, there is now positive evidence against a Middle Eocene 
communication between South America and Africa north of the equator. M. de 
Lapparent sti!l thinks it probable that such a communication may have existed at 
that period, but it must have been further south; and he admits that this view is 
hypothetical and rests partly on negative evidence, partly on the peculiar character 
of the Eocene and Oligocene marine fauna of Patagonia, so different from that of the 
North Atlantic. Under the circumstances we cannot be too careful in making 
assertions. Dr, Pellegrin argues that we need not go back beyond the Middle 
Eocene for the appearance of the first Cichlids in Africa; for some time after, 
but not later than the Lower Miocene, Madagascar has been, temporarily at least, 
