156 EVOLUTION AND GENETICS 



climbed rocks or cliffs in order to glide or flap out over the water 

 as it dived for fish. 



Archaeopteryx. This pro-avian is represented by well preserved 

 fossils which give an excellent idea of its structure (Fig. 86). It 



Fig. 87. — Nestling hoatzin, climbing with thumb and forefinger. (From 

 Jungle Peace by William Beebe, published by Henry Holt and Company, 

 with the permission of author and publisher.) 



lacked a number of the characteristic flight adaptations of birds. 

 The bones, for example, were not hollow, and fusion had not oc- 

 curred to the extent noted in modern birds. The sternum lacked 

 the deep keel which provides attachment for the powerful flight 

 muscles of birds. The tail was not reduced to the rudder-like 

 condition found in modern birds, but consisted of a series of 



