2S2 Evolution and Distribution of Fishes 



A noteworthy feature, phylogenetically, is that the small 

 sub-family Homalopteridae has become environally adapted 

 to life in mountain-streams, from India to China and 

 Borneo. Evidently for the same reason as causes the 

 gradual absorption of the air-bladder in species that be- 

 come marine, namely the more perfect and rapid aeration 

 of the blood and the tissues, owing to life in constantly 

 agitated and oxygenated water, the air-bladder in these 

 becomes small or is entirely absorbed. Later we shall 

 see that a like principle always holds for other groups or 

 genera in alpine streams. 



The Siluridae, along with the derivative and highly 

 modified families Loricaridae and Aspredinidae, comprise 

 fully 1 200 species. By far the greater number of the 150 

 or thereby genera, consist purely of river and lake dwellers, 

 as were all of the fossil types at present known. But while 

 Arius of Eocene times was freshwater or possibly anadrom- 

 ous in one species, the existing species of it have become 

 marine, as have Diplomystes, Galeichthys, Felichthys, and 

 a few other genera, that are found mainly along the eastern 

 American coast. On the other hand, the group Plotosidae 

 of east Asia, evidently represents another and more recent- 

 ly evolved series, most of the species of which have become 

 marine shore-dwellers. 



The air-bladder in the Siluridae is not only as well de- 

 veloped as in Characinidae and Cyprinidae, it usually attains 

 even greater size and complexity than in these two families, 

 being often composed of three instead of two chambers, 

 and these may give off secondary lobes. In contrast to 

 this, and exactly simulating the sub-family Homalopteridae, 

 noted above, and the family Loricaridae to be noted below, 

 some like the sub-family Pygididae, have been carried up 

 with what are now high mountain streams of the Andes 

 and other S. American ranges as these underwent elevation. 

 So gradual reduction of the air-bladder has taken place till 

 in some [Cetopsis etc.) it is almost or altogether absorbed. 



The Loricaridae and Aspredinidae are highly modified 

 derivatives from the Siluridae, that include about 240 

 species, most of which are found in brawling mountain 

 streams and rivers of Central and South America. So, as in 



