78 EVOLUTION 



America, and Egypt there are to-day "mud-fishes," 

 with both lungs and gills, which breathe air during the 

 dry season. In India there is a perch (Anabas scandens) 

 that can live for days out of water though it has no 

 lungs and walk across rough country on its fins, or even 

 climb trees; and on the shore of the Indian Ocean are 

 other fishes (Periophthalmus) that remain on the shore 

 quite happily in search of food when the tide recedes. 

 These are admirable living illustrations of " the fish out 

 of water," and enable us to understand the Devonian 

 transition quite easily. The Devonian mud-fish, of 

 which we have fine fossil specimens, was in all essentials 

 similar to the modern Australian mud-fish. Probably its 

 lungs are merely an adaptation of a pair of floating 

 bladders to a new purpose. The fish rises and falls in 

 the water by compressing or expanding a bladder of air 

 in its interior. In some fishes the gas-bag is double, 

 and in others it is well supplied with blood-vessels, or 

 opens into the gullet. Here were the bags for air- 

 breathing provided in a rudimentary way, and selection 

 soon developed the most efficient. 



From this kind of fish to the amphibia is an easy 

 transition, and we are not surprised to find salamander- 

 like creatures in the Carboniferous rocks. The two 

 pairs of fins have now turned into short and clumsy 

 limbs a natural result of an animal walking on them 

 which terminate in broad webby feet with five toes. 

 " Some of them have so plainly the characters of their 

 fish ancestors with the beginning of the characters of 

 their reptile descendants that they form," Le Conte says, 

 "the most complete connecting link ever discovered." 

 Indeed, we have so remarkable a chain of forms connect- 

 ing the bird and the mammal with the fish through the 

 reptiles and amphibia that it affords ample compensa- 

 tion for the earlier obscurity of the ancestor of the fish. 



