212 FISHES. 



sometimes distant from eacli other a few miles only ; the inter- 

 vening space may have been easily bridged over for the 

 passage of fishes by a slight geological change affecting the 

 level of the watershed, or even by temporary floods ; and 

 a communication of this kind, if existing for a limited period 

 only, would afford the ready means of an exchange of a 

 number of species previously peculiar to one or the other of 

 those river or lake systems. Some fishes, provided with 

 gill-openings so narrow that the water moistening the gills 

 cannot readily evaporate ; and endowed, besides, with an 

 extraordinary degree of vitality, hke many Siluroids (Olarias, 

 Gallichthys), Eels, etc., are enabled to wander for some dis- 

 tance over land, and may thus reach a watercourse leading 

 them thousands of miles from their original home. Finally, 

 fishes or their ova may be accidentally carried by water- 

 spouts, by aquatic birds or insects, to considerable distances. 



Freshwater fishes of the present fauna were already in 

 existence when the great changes of the distribution of land 

 and water took place in the tertiary epoch ; and having stated 

 that salt water is not an absolute barrier to the spreading of 

 Freshwater fishes, we can now more easily account for those 

 instances of singular disconnection of certain families or 

 genera. It is not necessary to assume that there was a con- 

 tinuity of land stretching from the present coast of Africa to 

 South America, or from South America to New Zealand and 

 Australia, to explain the presence of identical forms at so dis- 

 tant localities ; it suffices to assume that the distances were 

 lessened by intervening archipelagoes, or that an oscillation 

 has taken place in the level of the land area. 



Dispersal of a type over several distant continental areas 

 may be evidence of its great antiquity, but it does not prove 

 that it is of greater antiquity than another limited to one 

 region only. Geological evidence is the only proof of the 

 antiquity of a type. Thus, although the Dipnoi occur on the 



