628 SCIENCE PROGRESS. 



of the collateral evidence which can be brought to bear 

 upon this question from sources other than that of the mor- 

 phology of the lake animals themselves. For although at 

 first sioht it might appear an easy thing, having obtained 

 these animals, to determine their actual affinities and thereby 

 decide once for all whether they are marine or not, the exact 

 reverse is actually the case. The more we know about these 

 animals the more difficult it becomes to determine what 

 their actual affinities really are, and recently quite a number 

 of alternative hypotheses have cropped up. Thus it is said 

 (i) that these animals are not marine at all, that they owe 

 their present character to the oceanic conditions operating 

 in a great lake ; (2) that they may be a common feature of 

 African lake faupas, and thereforesimply a unique character 

 of the African lake fauna as a whole ; (3) that their pecu- 

 liarities are due to the latitude in the environmental con- 

 ditions of the lakes themselves ; (4) that they are the 

 remains of a prehistoric /7'es/i zvater, possibly cretaceous 

 stock, which has survived in some of the African lakes ; 

 and so on, till the only satisfaction felt in the contemplation 

 of these solutions is the certainty that they cannot all be true. 



It is my chief object in the present article to examine 

 first the physiographical differences of the lakes them- 

 selves, so as to be able to form some sort of an estimate 

 of the effect these differences are likely to have produced 

 in the faunas of the different lakes, and, secondly, the 

 actual facts, so far as they have been at present ascertained, 

 regarding the distribution of the families and genera of the 

 aquatic animals among the lakes themselves. 



When the evidence relating to these two lines of inquiry 

 has been collected it will be seen that the greater number 

 of the foregoing hypotheses are clearly incapable of applica- 

 tion to the problems they were made to solve, and that 

 there remains a direct issue between the respective merits 

 of only three. Lastly it becomes apparent, when attempting 

 to decide which of these three residual hypotheses really 

 explains the nature and origin of these remarkable African 

 lake forms, that the evidence drawn from the morphology 

 of these animals themselves alone has any weight. 



