FAUNA OF THE AFRICAN LAKES. 



611 



discovered in Victoria Nyanza and the Niger. There are four 

 endemic species of fresh-water sponges. Only 5 groups of animals 

 contain no endemic types. 



Tanganyika is one of the most remarkable lakes in the world, 

 the only cases comparable being the Caspian Sea and Lake Baikal. 

 Recent figures are difiicult to ascertain, but while Baikal may 

 even surpass Tanganyika in the number of unique animal forms, 

 it appears that the Caspian is less striking. 



There is reason to believe that the relations between marine 

 and fresh -water organisms are intima.te and due to a community 

 of descent. The barriers which prevent a change of medium are 

 not insurmountable. Organisms originated in the ocean, and 

 have attained their distribution in fresh water in various ways. 

 Moore regarded many of the Tanganyika types as relicts from a 

 former ocean. 



The aquatic plants of Tanganyika are of less interest than the 

 animals. The higher plants show no peculiarities, but the Algae 

 differ from those of the remaining lakes. A. number of species 

 are endemic and others are usually marine or brackish in habit. 

 The phytoplankton is rich in species, and moi'e than 70 per cent, 

 of the forms do not occur in Nyasa or Victoria. 



Recent discoveries do not favour Moore's hypothesis of a 

 marine Jurassic origin for Tanganyika. Neither his comparison 

 of shells from the lake with marine fossil shells, nor his views on 

 the primitive nature of the halolimnic Gasteropods, have been 

 accepted by leading experts. No members of his halolimnic group, 

 save the Polyzoon and the medusa, can be regarded as peculiarly 

 marine. Tiie Polyzoon is allied to a species still living in Indian 

 seas and the medusa is known from other parts of Africa. The 

 endemic animal types are held to be specialised rathei- than 

 primitive in nature. Geological evidence is not more favourable. 

 The extensive beds of sandstone and conglomerate which occur 

 in the lake regions were probably formed under fresh-water 

 and terrestrial conditions. They are considered by some to be 

 of Triassic age, but may belong to a much earlier period, i. e. 

 Devonian. Thus there is no support for the view that the ocean 

 at one time extended over the Congo basin. Further, there is 

 much to show that the trough in which Tanganyika lies was not 

 formed until Middle Tertiary times. 



A comparison has been made between thalassoid shells from 

 Tanganyika and a fresh-water Cretaceous genus on the one hand 

 and fresh-water Pliocene shells on the other. Since the thalassoid 

 shells have been held to resemble types from such different sources, 

 they offer little indication as to the origin of the lake fauna. — The 

 quasi-oceanic conditions in Tanganyika may have produced an 

 effect on the organisms it contains. Germain asserts that 

 Gasteropods of marine aspect occur in other regions besides 

 Tanganyika-, and are derived from the fresh-water t^^pes of a, 

 former vast lake basin. On the present lakes becoming isolated, 



