Sauropsida : Class Aves 



The terms "lower" and "higher" are often applied to groups of 

 vertebrates. The Classes are usually mentioned in this sequence: fishes, 

 amphibians, reptiles, birds, mammals. The fishes and amphibians are 

 often referred to as "lower vertebrates." The implication is that mam- 

 mals are "highest." Without discussing the several possible meanings 

 of "low" and "high" as used in biology, it will suffice for the present 

 to say that they are commonly used to express degree of anatomic 

 specialization. There are certain broad and general features which are 

 common to all vertebrates, a "general body-plan" (Part I). Any 

 one of these features is subject to adaptive modification or "special- 

 ization." This is carried to a much greater extent in some animals than 

 in others. It will perhaps be not too great a feat of imagination to take 

 a dogfish, an amphibian necturus, a reptilian tuatara and, for a mam- 

 mal, even a cat (some other mammals would serve the purpose a little 

 better except that they are less familiar), and, after reducing them all 

 to the same size, cause them all to occupy the same space at the same 

 time in such a way that their corresponding outlines and shapes, both 

 external and internal, shall merge together and produce a composite 

 or average of the several animals. This may be done, in imagination, 

 without extremely violent distortion of any one of the subjects. But 

 take, instead of a dogfish, a sea horse (Fig. 337) together with a toad, 

 a snake or turtle, and an elephant. To imagine a blend or composite of 

 this series is much more difficult because each of the animals has, in 

 some respects and in very different ways, gone to extremes of anatomic 

 specialization. 



Whether a bird is more specialized than a snake, a whale, or an 

 elephant is perhaps open to debate Some birds are, in some ways, more 

 specialized than others, but it is certainly true that all birds are very 

 highly specialized. In each of the other Classes, except Cyclostomata, 

 there are important groups which, relative to others in the Class, are 



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