CHAP. XXIII.] SUMMARY AND CONCLUSION. 549 



Fresh-water Fishes. 



Although it would appear, at first sight, that the means of 

 dispersal of these animals are very limited, yet they share 

 to some extent the wide range of other fresh-water organisms. 

 They are found in all climates ; but the tropical regions are 

 by far the most productive, and of these South America 

 is perhaps the richest and most peculiar. There is a certain 

 amount of identity between the two northern continents, and 

 also between those of the South Temperate zone ; yet all are 

 radically distinct, even North America and Europe having but 

 a small proportion of their forms in common. The occurrence 

 of allied fresh-water species in remote lands — as the Aphritis 

 of Tasmania and Patagonia, and the Comephorus of Lake 

 Baikal, distantly allied to the mackerels of Northern seas — 

 Avould imply that marine fishes are often modified for a life in 

 fresh waters ; while other facts no less plainly show that per- 

 manent fresh-water species are sometimes dispersed in various 

 ways across the oceans, more especially by the northern and 

 southern routes. 



The families of fresh-water fishes are often of restricted 

 range, although cases of very wide and scattered distribution 

 also occur. The great zoological regions are, on the whole, very 

 well characterized ; showing that the same barriers are effectual 

 here, as with most other vertebrates. "We conclude, therefore, 

 that the chief lines of migration of fresh-water fishes have been 

 across the Arctic and Antarctic seas, probably by means of float- 

 ing ice as well as by the help of the vast flocks of migratory 

 aquatic birds that frequent those regions. On continents they 

 are, usually, widely dispersed ; but tropical seas, even when of 

 small extent, appear to have offered an effectual barrier to their 

 dispersal. The cases of affinity between Tropical America, 

 Africa, Asia, and Australia, must therefore be imputed either to 

 the survival of once widespread groups, or to analogous adap- 

 tation to a fresh-water life of wide-spread marine types ; and 

 these cases cannot be taken as evidence of any former land 

 connection between such remote continents. 



