EVOLUTION OF THE VERTEBRATES 157 



vertebrae and bore lateral feathers. The wings were w(^ll de- 

 veloped, but the first three digits remained free as long clawed 

 grasping appendages. Finally the jaws bore teeth, though this 

 condition was found in other extinct birds as well. Archaeoptcryx 

 is therefore not truly a bird, and yet is so definitely bird-like as to 

 be beyond the reptilian ancestor whose discovery is still to be 

 made. 



The Hoatzin. One existing species of bird is reminiscent of this 

 ancient pro-avian condition. This is the hoatzin of tropical South 

 America. In young hoatzins the first and second digits, unlike the 

 vestigial remnants found in other birds, are functional and are used 

 effectively as grasping organs in climbing (Fig. 87). The young 

 hoatzin is also an able swimmer. 



Mammalian Evolution. The mammals differ less conspicu- 

 ously from the reptiles, since their development is not associated 

 with any extreme specialization. The skeleton shows certain 

 peculiarities, including triple centers of ossification in long bones 

 and paired occipital condyles. Hair, a characteristic mammalian 

 structure, has been definitely associated with scales in proof of 

 transition between the two (Fig. 58). The teeth undergo great 

 differentiation and are of several kinds, i.e., the dentition is 

 heterodont. Both the process of reproduction and the nourish- 

 ment of the young by milk, a glandular secretion, are generally 

 distinctive, but the lowest mammals, the Prototheria or Mono- 

 tremata, lay eggs. In many ways the mammals are similar to the 

 birds in degree of development, and they correspond closely in the 

 four-chambered heart and vaso-motor apparatus, but in most 

 respects the two classes are divergent. 



Mammalian Ancestors: The Cijnodonts. Because of structural 

 peculiarities the mammals were once supposed to be more closely 

 related to the amphibia than to the reptiles, but the evidence of 

 paleontology now points definitely to the Cynodontia, the dog- 

 toothed reptiles, as their ancestors. 



The cynodonts are distinctly reptilian animals in most particu- 

 lars but they agree with the mammals in two distinctive ways. 

 The skull is articulated with the first vertebra V)y two occipital 

 condyles instead of one as in other reptiles, and the dentition is 

 heterodont, while in other reptiles all of the teeth are primitive 

 conical structures. 



Among the cynodont characteristics which are favorable for 



