632 SCIENCE PROGRESS. 



Carefully bearing In mind the above enumerated facts, 

 we may now examine the actual distribution of animals 

 amongst the African lakes so far as this distribution is at 

 present known, attempting thereby in the first place to 

 estimate how far the individual variations presented by the 

 faunas of the different lakes may be due to differences of 

 environmental condition, or to what extent they must be 

 explained through the operation of some other cause. 



The Molluscs of the individual lakes are the group which 

 is at present by far the most widely known, and the Mol- 

 luscs from different localities show specific differences, which 

 are at once obvious to the most casual observer ; in fact 

 very little experience of the Molluscs of different lakes 

 enables one to tell at a glance from which lake any j)ar- 

 ticular species may have come. Thus the Paludinas, which 

 abound in the most astonishing profusion in nearly all the 

 African lake waters, whether salt or fresh, show all sorts of 

 gradations in size, from the little variety found in the fresh 

 water of Nyassa to the large angular species of Mwero, and 

 lasdy to the largest Neothaumas of Lake Tanganyika, 

 which species increases in size from the south to the north 

 of the same lake. What is true of the Paludinas is true of 

 the other Molluscs. There are specific differences peculiar 

 to each lake, but beyond this the differences between the 

 lakes is generally apparent simply in the omission of more 

 or fewer genera as we pass from the great lakes to the 

 small. The larger lakes have invariably by far the 

 greater number of different genera, not only of Molluscs, 

 but also of fish, Crustacea, sponges, and the like. Nyassa, 

 which ranks third in actual dimensions, and which, owing to 

 its enormous depth, probably contains more water than any 

 other single African lake, has yielded about seventeen 

 different genera of Molluscs, among which there are the 

 following genera of Gastropods : Limnea, Isodora, Physop- 

 sis, Planorbis, Ampullaria, Lanistes, Vivipera, Cleopatra, 

 Bythinia, and Melania. Except that of Tanganyika, the 

 Molluscan fauna of Nyassa is the largest of any known 

 African lake, and it will be seen from the above list of 

 Gastropods that it is, so far as they are concerned, of an 



