102 MEMOIRS OF THE CARNEGIE MUSEUM 



B. The same facts also dispose of the notion that this fauna as a whole may 

 have developed concomitantly with the lowland fauna from a common nucleus. If 

 it had so developed, we could not expect so large a percentage to be identical. 



C. Is this poverty-stricken fauna the relict of a more abundant and divergent 

 fauna, dating from a time when the plateau may have been easily accessible to the 

 present diverse fauna of the lowland? 



If it is such a relict the fact implies: 



(a) That at some time in the past, when the plateau was more easily acces- 

 sible than now, in fact formed part of the general level, it shared the fauna of the 

 region with the general level. 



(b) That the fauna became isolated by the gradual elevation and formation 

 of the plateau, and of the falls in its rivers. 



A simple isolation will not account for all the facts. To account for the 

 poverty we must assume either that the isolation took place before the origin of 

 the present variety in the lowland, or that conditions altered so that most of 

 the great variety of the lowland became extinct. The first horn of this dilemma 

 assumes that the fauna as a whole is a relict of the original fauna, which we have 

 shown to be most improbable. I am not prepared to satisfactorily deal with the 

 other horn of the dilemma. The elevation at the present time is not sufficient to 

 exclude these various lowland types. I do not know of any evidence that other 

 unfavorable conditions have obliterated the fauna of the plateau. 



D. Does the fauna consist of recent immigrants? 



If it consists of recent immigrants only, we must assume that the plateau 

 became isolated before the origin of the fauna of the lowland, or again, that at 

 some time it became elevated to such a height that practically the entire fauna 

 was wiped out, and later restored by immigrants. Assuming that one or the other 

 of these alternatives is a fact, we may discuss the possibility of the immigration of 

 species. 



In this connection the word "migration" as applied to fishes needs defini- 

 tion. It is applied to such journeys as that of the eel to the ocean, or that of the 

 salmon toward the headwaters, for purposes of reproduction. Such trips, although 

 very long, probablj^ only incidentally influence the dispersal of fishes. On the 

 other hand, some species are natural pioneers, found always in the rivulets of the 

 headwaters, as far up as they can get. If the headwater is advanced a few feet, they 

 advance with it. Their migration from one system to another is not miraculous. 

 To find such fishes on opposite sides of a low water-shed presents no mystery. If 

 by a sudden freshet two rivulets join for but a short time, these species are present 



