Facta and Suppositions. 11 



This argument cannot be fully appreciated by those who 

 are not extensively acquainted with natural history, but we 

 may, perhaps, make it plainer by alluding to some other 

 similar facts. Our fresh waters teem everywhere with 

 animals and plants. Fishes and mollusca are among the 

 most prominent of their animals. Let us compare for a 

 moment tHe diflPerent species which occur in the Danube, in 

 the Rhine, and in the Rhone, three hydrographic basins en- 

 tirely unconnected with each other throughout their whole 

 extent. They spring from the same mountain chain, as we 

 may take the Inn as the source of the Danube. These three 

 great rivers rise within a few miles of each other. Neverthe- 

 less, most of their fishes differ, but there are some which are 

 common to the three. We find the pickerel, — the European 

 pickerel, in the three basins. The eel is also common to 

 them all. One kind of trout occurs in the three. But how 

 strange the distribution of some others ! — ^for instance, the 

 perches. In the Rhine we find Perca fluviatilis^ and Acerina 

 cernua ; in the Rhone, Perca fluvia tills ^ and Aspro vulgaris ; in 

 the Danube, Perca vulgaris^ Lucio-perca Sandra, Acerina cer- 

 nua, A. Schraitzer, Aspro vulgaris, and A. Zingel. If these 

 animals had not originated in these rivers separately, why 

 should not such closely-allied species, some of which occur in 

 the three basins, have all spread equally into them 1 and if 

 they originated in the separate basins, we have within close 

 limits a multiple origin of the same species. 



And that this multiple origin must be admitted as a fact 

 is shewn by the following further evidence. Among the 

 carpes we find, for instance, Barbus, Gobio, Carpio, common 

 to the three. But the Danube has three Gobios, whilst the 

 others have but one, one of the Danube being identical with 

 the one of the other two rivers. The most striking fact, 

 however, occurs in the genus Leucisciis. Leuciscus dobula is 

 common to the three ; but in addition to it, the Danube has 

 several species which occur neither in the Rhine nor in the 

 Rhone. The basin of the Rhone, again, has several species 

 which occur neither in the Danube nor in the Rhine ; and in 

 the Rhine, there are species which belong neither to the 

 Rhone nor to the Danube. Now, we ask, could all these 



