88 SCIENCE SKETCHES, 



most rivers flowing under similar circumstances upon the 

 surface of the globe. Nothing, however, short of such 

 collections, compared closely with one another, will fur- 

 nish a reliable answer. . . . Whoever will accomplish 

 this survey will have made a highly valuable contribution 

 to our knowledge." 



Certain conclusions were also suggested by 

 Professor Cope in his excellent memoir on the 

 fishes of the Alleghany region ^ in 1868. From 

 this paper I make the following quotations : — 



"The distribution of fresh-water fishes is of special 

 importance to the questions of the origin and existence of 

 species in connection with the physical conditions of the 

 waters and of the land. This is, of course, owing to the 

 restricted nature of their habitat, and the impossibility of 

 their making extended migrations. With the submer- 

 gence of land beneath the sea, fresh-water fish are de- 

 stroyed in proportion to the extent of the invasion of salt 

 water, while terrestrial vertebrates can retreat before it. 

 Hence every inland fish-fauna dates from the last total 

 submergence of the country. 



'' Prior to the elevation of a given mountain chain, the 

 courses of the rivers may generally have been entirely 

 different from their later ones. Subsequent to this period, 

 they can only have undergone partial modifications. As 

 subsequent submergences can rarely have extended to 

 the highlands where such streams originate, the fishes of 

 such rivers can only have been destroyed so far as they 

 were unable to reach those elevated regions, and preserve 

 themselves from destruction from salt water by sheltering 

 themselves in mountain streams. On the other hand, 



1 On the Distribution of Fresh-Water Fishes in the Alleghany 

 Region of Southwestern Virginia. Journ. Acad. Nat. Sci., Phila., 

 1868, pp. 207-247. 



