THE AQUATIC VERTEBRATKS 1037 



Others of undoubted marine origin have entered fresh water at 

 such remote periods that they have set up distinct fresh-water 

 families, as the suntishes, the perches, and the CichHdac. 



Finally, we have the dominant fresh-water groups of characins, 

 minnows, carps, suckers, and catfishes whose origin from the 

 sea is so remote that the orders and superorders embracing all of 

 these dominant members of the fresh-water fauna, with the excep- 

 tion of Arius and related genera, are peculiar to fresh water. 



Fig. 1539. Miller's Thumb, Coitus ictalops (Rafinesque). 



Dispersal of Fresh-water Fishes. — No fishes have been or are 

 bemg permanently contributed to the land. The eel is capable 

 of moving over short stretches of land, and Periophthalmus may 

 leave the water in search of food. In the South American fresh 

 waters a relative of the catfish is said to be able to move from 

 pond to pond, and in the Congo and in South American rivers 

 live fishes that temporarily fly over the water. But all these spe- 

 cies are adapted to the water and can live for longer periods only 

 in connection with it. 



The two factors that more than others are responsible for the 

 abundance or paucity of the faunas are accessibility and tempera- 

 ture. The latter will be considered more at length later. Acces- 

 sibihty demands some attention now. 



A locality is accessible to fishes if it is connected with an inhab- 

 ited locality by a permanent or seasonal waterway. There are 

 fishes that apparently defy tliis general rule and that skip or have 



