THE FAUNA OF THE GREAT AFRICAN LAKES. 631 



eventually reaching Tanganyika through the Luakuga, is, 

 and must for a long time hav^e been, the most impassable 

 of the lot. 



It is thus apparent that the general physiographical 

 characters of the African lakes, the nature of the water in 

 them, as well as the characters of the effluents which flow 

 out of them when such exist, are in no way similar, they are 

 on the contrary remarkably diverse, the physiographical 

 characters of the lakes being, in fact, far more unlike each 

 other than any one who was unacquainted with the wide 

 climatic variations induced by altitude in the tropics would 

 be likely to suppose. Such diversity in the environments 

 to which the faunas of the great African lakes must have 

 been, and are now exposed, would, I imagine, be regarded 

 by all experienced naturalists as quite sufficient if this action 

 was at all prolonged, to induce profound changes in the 

 characters of the organisms which originally migrated into 

 them ; but I know of no method by which the effect pro- 

 duced by environmental difference can be foretold, all that 

 can be done is to examine the faunas of the lakes themselves 

 collectively, putting down those individual peculiarities they 

 present, which do not appear to be produced by oceanic 

 contamination, or some such entire difference of nature to 

 the operation of this cause. It must however be borne in 

 mind that the power of changed environment, to produce 

 corresponding changes in animal organisation, is by no 

 means unlimited, and certainly its effect in Africa is more 

 often witnessed in the reduction of the number of genera in 

 a lake, than in the production of new ones. Exceptional 

 conditions appear to be adverse to fauna as a whole, and in 

 no way productive of abundant variations. Change in their 

 environment however is not the only factor which might, 

 and probably has, operated in differentiating these African 

 faunas one from another. As Darwin pointed out many 

 years ago it is only necessary to run a fence across a cattle 

 park to produce different breeds on either side of it, and 

 the isolation of groups of animals to which this process is 

 practically equivalent, has been carried to a high degree 

 of perfection in many of the African lakes. 



