292 OUTLINES OF EVOLUTIONARY BIOLOGY 



Devonian period and to have attained their maximum of 

 development and specialization during that epoch. This group 

 is of especial interest, as we have already had occasion to notice, 

 as indicating how the transition from an aquatic to a terrestrial 

 mode of life was first rendered possible to vertebrate animals by 

 the conversion of the swim-bladder into a pair of lungs. The 

 dipnoids, however, have remained aquatic in habit, and retain 

 their gills for aquatic respiration as well as being able to breathe 

 air directly by means of their lungs. 



The ordinary bony fishes (Teleostei), to which the vast 

 majority of existing species belong, do not attain any prominence 

 as a group before the secondary era, but they had fore-runners 

 in some of the early ganoids which date back to lower 

 Devonian times. The teleosts are the most specialized of all 



FIG. 138. Bestoration of Acanthodes ward!, from the Coal Measures of 

 Staffordshire, X \. (From British Museum Guide.) 



fishes and it is not from such a group that we should expect the 

 next great advance in organization to take its origin. 



There are the strongest anatomical and embryological grounds 

 for believing that the Amphibia, a group which at the present 

 day includes the frogs, toads, newts and salamanders, are 

 the descendants of fish-like ancestors which became adapted to 

 an air-breathing mode of life by the development of lungs. The 

 dipnoid fishes, which, as we have already seen, attained their 

 maximum of development in the Devonian epoch, show us 

 clearly enough how such a transition probably took place, and 

 it was doubtless either from some dipnoid form, or from some 

 other primitive type of fish which had also succeeded in con- 

 verting its swim-bladder into lungs, that the Amphibia arose. 

 The fact that amphibian remains first occur in Lower Carboni- 

 ferous rocks is in complete harmony with this hypothesis as to 

 the origin of the group. 



