THE AQUATIC VERTEBRATES IO39 



from the Neuse to the Santee, by descending to the lower courses 

 of the rivers, and thence passing along either through the swamps 

 or by way of the sea. The lower courses of these streams, warm 

 and muddy, are uninhabitable by such fishes. Such transfers are, 

 however, possible farther north. From the rivers of Canada and 

 from many rivers of New England the trout does descend to the 

 sea and into the sea, and farther north the white fish does this also. 

 Thus these fishes readily pass from one river basin to another. As 

 this is the case now everywhere in the north, it may have been the 

 case farther south in the time of the glacial cold. We may, I 

 think, imagine a condition of things in which the snow fields of 

 the Allegheny chain might have played some part in aiding the 

 diffusion of cold-loving fishes. A permanent snow field on the 

 Blue Ridge in western North Carolina might render almost any 

 stream in the Carolinas suitable for trout, from its source to its 

 mouth. An increased volume of colder water might carry the 

 trout of the head streams of the Catawba and the Savannah as 

 far down as the sea. We can even imagine that the trout reached 

 these streams in the first place through such agencies, though of 

 this there is no positive evidence. For the presence of trout in 

 the upper Chattahoochee we must account in some other way . . . 



"With the lowland species of the southern rivers it is different. 

 Few of these are confined within narrow limits. The streams of 

 the whole South Atlantic and Gulf Coast flow into shallow bays, 

 mostly bounded by sand pits or sand bars which the rivers them- 

 selves have brought down. In these bays the waters are often 

 neither fresh nor salt; or, rather, they are alternately fresh and 

 salt, the former condition being that of the winter and spring. 

 Many species descend into these bays, thus finding every facility 

 for transfer from river to river. There is a continuous inland 

 passage in fresh or brackish waters, traversable by such fishes, 

 from Chesapeake Bay nearly to Cape Fear; and similar con- 

 ditions exist on the coasts of Louisiana, Texas, and much of 

 Florida." 



Adaptations to the Main Object in Life. — Fishes either lay 

 eggs that are fertiKzed in the water, retain eggs that have been 

 internally fertilized to the time of hatching or in a few species give 



