432 



THE CHANGING GENERATIONS 



only in the lesser development of the limb bases. Why did the amphibians 

 leave the water? Not to breathe air, for that could be done by rising to 

 the surface of the pool; not in search of food, for there was little food for 

 carnivores on land; and not to escape enemies, for they were among the 

 largest inhabitants of the fresh waters. Instead, their appearance on land 

 was probably an adaptation for remaining in water. So long as there 



remained any water in the pools, 

 the lobe fins were probably the 

 better off of the two groups, for 

 they were better swimmers; but 

 when the pools dried up completely 

 the lobe fins had to burrow into 

 the mud and would die if the 

 drought were prolonged, while the 

 ancestral amphibian could crawl 

 out of the pool and walk overland 

 on his stumpy leg fins to the next 

 pool where water still remained. 

 Once this habit had been formed, a 

 land vertebrate fauna could be 

 built up. Instead of seeking water 

 immediately, the amphibian might 

 linger on the banks and devour 

 stranded fish; some might take to 

 eating the primitive insects already 

 present on land, and the larger 

 ones might gradually come to prey 

 on their smaller amphibian rela- 

 tives. Although a true land fauna 

 could thus be established, the 

 amphibians remained tied to the 

 water by their reproductive require- 

 ments. We can be certain that their 

 eggs were laid in water and extern- 

 ally fertilized, and fossils prove that 

 the young passed through a fishlike aquatic stage, as do those of most 

 modern amphibians. 



The dominance of the amphibians. For a geologically brief time the 

 amphibians were the rulers of the lands. During the late Paleozoic they 

 increased rapidly and branched out into many types adapted to different 

 modes of existence — a phenomenon known as adaptive radiation, which 

 has occurred in group after group as each attained temporary dominance 

 in the world of life. Some of the Paleozoic amphibia were small, salaman- 



Fig. 27.17. From fish to amphibian. Above, 

 the Devonian lobe-finned fish Eusthenop- 

 teron leaving the water to sprawl about on 

 its stout, muscular fins. Below, one of the 

 earliest known land vertebrates, the "stego- 

 cephalian" amphibian Diplovertebron of 

 the lower Carboniferous. {Courtesy Ameri- 

 can Museum of Natural History.) 



