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. 
