CHAPTER VII 



AVES 



The birds (Aves) are feathered vertebrates; or, as Huxley ex- 

 pressed it, ''glorified reptiles". The class evolved from the diap- 

 sid reptiles, but are far more specialized than any living rep- 

 tilian group. In fact, it is the most coherent class of vertebrates; 

 and when considered as an entire class, is the highest major 

 group of the vertebrates. ]\Ian and certain other mammals sur- 

 pass them in the specialization of the nervous system, although 

 the average of the birds is almost as high as that of the lower 

 mammals. 



Birds are warm blooded and have a constant temperature; the 

 skeleton is light and strong, a modification in correlation with 

 their flying habits; the heart is completely four-chambered like 

 the mammals; and the fore limbs are highly modified from the 

 primitive arm of the land vertebrates. 



Little is known about the evolution of the flying mechanism 

 in birds. Two fossils from the lithographic limestone of Germany 

 tell something as to the evolution of the birds from the reptiles, 

 but it is still in doubt as to how flying originated. Various the- 

 ories have been advanced. The fossils from Germany are called 

 Archaeopteryx, or "ancient wing", and these fossils indicate that 

 the early birds were not powerful fliers. Study of the illustration 

 will show a number of interesting features. The skull is heavy 

 and reptilian, not light as in living birds; there are conical teeth, 

 much like those of a reptile; the digits of the wing are well devel- 

 oped with claws at the end; and the tail is long like that of a 

 lizard, but with a row of feathers on either side. Evidently this 

 animal was hardly more than a feathered reptile. 



Two later fossil birds are known, which are more distinctly 

 bird-like in their anatomy. The tail, wing, and hind leg had be- 

 come modified to the avian condition before the teeth were lost 



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