CHAPTER XIV 



BIRDS AND EVOLUTION 



§ I. The Fact of Evolution. § 2. Factors in Evolution. § 3. Origi- 

 native Factors. § 4. Illustrations of Variations. § 5. Possible 

 Origins of Variations. § 6, Modifications in Birds. § 7. Modifi- 

 cational Specific Characters. § 8. Directive Factors : Processes 

 of Selection. § 9. Isolation and the Reverse: In-breeding and 

 Out-breeding. § 10. Birds and Heredity. 



§ I. The Fact of Evolution 



It is the almost unanimous conclusion of modern naturalists 

 that Birds sprang from some extinct stock of Reptiles, 

 probably from the Ornithischian Dinosaurs. There are 

 such very striking contrasts between the reptiles, so cha- 

 racteristically bound to the earth, and the birds which have 

 achieved the mastery of the air, that the derivation of the 

 latter from the former seems at first sight startling. We 

 venture, therefore, to reinforce what has been said in the 

 preceding chapter. What are the main lines of evidence ? 



(a) Morphological. — The bird betrays its reptilian origin 

 in many a corner of its body, from the epidermic scales on 

 its bill to those on its feet, from the complex lower jaw 

 working on the quadrate to the inter-tarsal ankle-joint, from 

 the sclerotic ring of bone round the front of the eye to the 

 egg-tooth with which the chick breaks its way through the 

 egg-shell. For all reptiles show epidermic scales and a 

 complex lower jaw working on a quadrate ; all that have 

 hind-legs show an inter-tarsal ankle-joint ; and some of 

 them show a sclerotic ring and an egg-tooth. Unless the 

 processes of Nature are magical, the possession of many 

 structural features in common, for we have given only a few 

 instances, must mean genetic affiliation ; and the rock record 

 shows that Reptiles appeared before Birds. 



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