606 



DR. W. A. CUNNINGTON ON THE 



though the genus Gammarus is I'ecorded from, the north and 

 south of the continent. Why such types are wanting it would 

 be idle to speculate, but the fact is also emphasised by Stuhlmann 

 in more than one place (181, p. 1268 : 182, p. 652). It is strange 

 too, that among the intestinal parasites of the fresh-water fish 

 there appear to be no species of Echiitorhynchus, although they 

 are common in the fish of European rivers, Daday, it is true, 

 has described a larval form from a species of Ostracod taken in a 

 small East African lake (76, p. 55). Thus, while Asellus and 

 Gammarus are commonly the intermediate liosts, it is evident 

 that the absence of these genera does not form a complete 

 barrier to the distribution of Echinorliynclius^ and Daday con- 

 siders that it will yet prove common in Africa. At present, 

 however, the adult form is entirely unknown. 



Concerning the numerical distribution of species in the lakes, 

 there is one point which calls for further notice. From a study 

 of the lists of forms found in each lake, Moore believed that a 

 definite relation existed between numbers and size. After 

 reviewing the facts then at his disposal, he writes : — It is thus 

 obvious that from some cause or other the number of specific 

 forms in an African lake is roughly proportional to the size of the 

 lake itself" (137, p. 146). This does not mean that the smaller 

 lakes are less well stocked with animals, but simply that the 

 number of species they contain is less. The matter has already 

 been referred to in the systematic section, and it has moreover 

 been shown that the principle appears capable of extension to the 

 number of genera and even families (pp. 535, 548). With the 

 tofcal figures for the six lakes now available, it is possible to test 

 the correctness of this conception on a more extended basis. 

 The totals for species and genera are therefore given in tabular 

 form, with the lakes (apart from Tanganyika) arranged in order 

 of size. 



Taiigan- Victoria -j^ ^^^^^ Albert Edward YJvsVi 

 yika. Nyanza, Nyanza. Nyanza, 



Number of Species 402 289 361 67 54 23 



Genera 168 139 178 48 35 13 



Tanganyika, which heads the list, is in every sense to be 

 regarded as a case apart, but the figures for the remaining lakes 

 should accord with this law — if such it be. A descending series 

 is seen to exist : Nyasa it is true constitutes an exception, but, 

 as already explained, its totals have been artificially swollen in 

 certain directions (p. 595). Nyasa conforms to the rule in the 

 case of the Pisces and MoUusca — it is the addition of many types 

 of Hotifera and Protozoa which chiefly accounts for the large 

 total figure for the lake. 



