420 Evolution and Distribution of Fishes 



the other two lateral and paired. In Homalopterinae the 

 small air-bladder consists of paired halves with bony en- 

 closure. The reduced air-bladder that in some is almost 

 absorbed, appears to result, as already stated (p. 352) 

 from the group having become inhabitants of mountain- 

 streams, in which rapid oxygenation of the blood and tissues 

 is effected, so that use of the air-bladder can be dispensed 

 with. 



From the geological standpoint then, the united testi- 

 mony of fossil and recent genera of the Cyprinidae is that 

 the family started in N. America, probably in the late 

 Cretaceous period, and with forms referable to the simplest 

 or catostomid section. Extending widely over the northern 

 continent, they evolved numerous cyprinid descendants by 

 early Eocene days, but they were prevented reaching S. 

 America, owing to discontinuity between the two continents 

 in the late Eocene and in Miocene times. Favored by a wide 

 western connection with Angara-land, the now abundant 

 species of Cyprinids extended into that land, along with a 

 few catostomid forms, and these spread westward as well as 

 southward, in the latter case giving rise to genera that be- 

 came subtropical and later tropical in environment. These 

 overran the Angara continent to its western limit, reaching 

 even to central and south Europe by late Oligocene days. 



An equally strong extension across N. Atlantis ulti- 

 mately met the above in derivative species of each; while 

 the Lobotinae or Cobitidinae, coming off as a sideline from 

 east Asiatic Cyprinids, overspread Asia and Europe, with 

 an outlier along the Nile valley, in relatively recent periods. 

 From a derivative type, intermediate between the Cyprini- 

 nae and Lobotinae the Homalopterinae probably arose. 

 And it seems not unlikely that the mountain types, which 

 mainly compose the last group, may, through gradual fold- 

 ing and upheaval of the Himalaya — Malaya ranges, have 

 acquired their present peculiarities, owing to proenvironal 

 adaptations to new and changing environal stimuli that 

 started when the mountains were upheaved to their final 

 limit, during the Pliocene period. 



The Characinidae closely suggest geologic and geo- 

 graphic connections similar to those that permitted distri- 



