792 Comparative Morphology of Chordates 



sumably also lung-breathing, is a matter of opinion. That Dipnoi and 

 Amphibia had common origin from erossopterygians is perhaps the 

 safest view. Having started with a hypothetic primitive chordate, we 

 must again draw on imagination to get a finned fish out onto land as a 

 tetrapod amphibian. No "connecting links" are known, but the fin- 

 skeleton of some ancient erossopterygians may, without too great a 

 strain on imagination, be converted into the skeleton of a leg (Fig. 123). 

 Allowing a million years, or more if necessary, and admitting that we 

 know not how the changes could have taken place (nor do we fully 

 know the mechanism of any other evolutionary change), we can only 

 imagine that some shallow-water fishes gradually shifted their habitat 

 from water (presumably fresh water) to land, their fins meanwhile 

 becoming legs. 



Amphibians, restricted to life near water, never as such achieved 

 wide distribution and dominance, but from them descended animals 

 which, by appropriate changes in skin, respiratory organs, and method 

 of reproduction, were able to free themselves from the ancient bonds 

 of water and became wholly terrestrial reptiles. Among late Paleozoic 

 tetrapod vertebrates were some (e.g., Seymouria) which, as judged by 

 fossil skeletons, might have been reptile-like amphibians or amphibian- 

 like reptiles. Once having attained a foothold on land and freedom to 

 migrate inland, the tetrapod vertebrates found themselves on the 

 threshold of an era of such expansion, diversification, and increase in 

 size and power as gave them mastery of the habitable land surfaces of 

 the Mesozoic world. The diversification even went so far that some 

 reptiles reverted to acpiatic living (but remained air-breathing), and 

 others acquired power of flight. It was an Age of Reptiles. 



Among early reptiles were some whose skeleton and teeth exhibit 

 mammalian characteristics. Evidence of transition from reptiles to 

 mammals is quite as satisfactory as that for origin of reptiles from 

 amphibians. The early mammal-like (theromorph) reptiles lasted for a 

 relatively short time, but it was long enough for them to give rise to a 

 line of descendants which were unmistakably mammalian. Down 

 through one or two hundred million years of the reptilian world trickled 

 a thin stream of small (compared to reptilian contemporaries) and 

 obscure mammals. It is possible that their smallness and obscurity 

 favored their survival. When, in the later Mesozoic, climatic changes 

 or other circumstances brought on hard times for the reptiles, the 

 mammals, by dint of their warm blood, fur, and potentially superior 

 neuromuscular equipment, began to come into their own and eventu- 

 ally acquired the dominant position which they have retained down to 

 the present. 



Meanwhile, birds appeared. As a "connecting link" between 



