GENERAL ZOOLOGY 



restrial vertebrates were Amphibia, and they arose from lobe-finned fishes 

 (Fig. 18.11). It will be recalled that these fishes were already provided 

 with lungs; the bones of their fins are closely comparable with those of the 

 limbs in amphibians. It is from these primitive four-footed land animals that 

 the descent of all the later terrestrial vertebrates may be traced. If we put 

 together what we know from the fossils and from the geological formations, it 

 seems probable that the factors influential in the change of habitat from water 

 to land, and in the evolution from four-finned to four-footed vertebrates, were 

 such that early land animals were led to seek water rather than land. 

 Droughts appear to have been frequent in the Devonian; it is thought that 

 forms capable of moving across countrv "when the food failed and the last 

 water dried" survived, whereas those that persisted as lobe fins gradually 

 became extinct. Whatever may have been the course of events, the changes 

 in structure and in habitat were eflTected. With the spread of these early 

 terrestrial forms and the decline of the fishes came the Age of Amphibians, 

 the first act of the evolutionary drama during which the four-footed verte- 

 brates came to possess the land. 



The Amphibia. Most amphibians of the present day remain half water 

 dwellers and half land dwellers; they lead a "double life," as their name 

 indicates. Although amphibians typically have well-developed lungs, these 

 organs are supplemented by "skin breathing" in most species. The tadpole 

 stage of amphibians is fish-like, not only in structure but also in habitat and 

 activities. The earliest amphibians available as fairly complete skeletons 

 resemble both the lobe-finned fishes and the reptiles. It appears that the 

 early amphibians gave rise to certain specialized types which became extinct, 

 to the ancestors of modern amphibians, and to the ancestors of reptiles. 

 Among the primitive types were those called Stegocephalia, or "roof-headed," 

 because of their heavily armored skulls (Fig. 18.13). Other forms were lean 

 and active swimmers, as indicated by the characteristics of their skeletons. 

 The conspicuous dermal plates, like those of fishes and reptiles, which were 

 present in many early amphibians, have disappeared in modern forms. The 

 limbs were used in locomotion, but they did not carry the weight of the body, 

 which still trailed along the ground and probably aided in movement by 

 sinuous undulations. The separation of oxygenated from unoxygenated blood 

 in the circulatory system was presumably incomplete, as in the amphibians 

 of today (p. 52). With the divergence of the reptilian line, the extinction of 

 highly specialized types, and the difTerentiation of the surviving lines, the 

 evolution of the modern forms was well on its way when the Age of Am- 

 phibians drew to a close. The flourishing of amphibians was probably cor- 

 related with the absence of other large land animals as competitors and with 

 the swampy conditions that prevailed in Carboniferous times. Toward the 

 end of the Permian, competition with the reptiles, a type better adapted for 

 terrestrial life, and the advent of drier terrestrial conditions, were perhaps 

 important factors in the decline of the amphibians. In any event, the Am- 

 phibia of today are small in size and few in number by comparison with those 



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