CHAPTER XL 



THE EVOLUTION OF THE BIRDS 



The birds present so many similarities to the reptiles that they 

 have been classified together with them in the group Sauro- 

 psida. The resemblances extend to the following features. 

 The heart and arteries of the bird are the same as those of the 

 crocodile with the exception of the left systemic arch, which 

 in birds is abolished. The perivisceral coelomic cavity of 

 birds is divided up into pulmo-hepatic recesses and pleural 

 cavities, by means of the pulmo-hepatic ligaments and oblique 

 septa ; this arrangement is also present in the crocodile. The 

 lung of birds gives rise to a number of diverticula or air-sacs 

 which ramify about inside the body ; small air-sacs are formed 

 by the lung of the chamasleon. With regard to the nervous 

 system, the brain of birds is an elaboration of the grade of 

 structure shown by the brain of crocodiles, and its distinctive 

 feature is that the corpus striatum has been especially developed 

 while the cerebral cortex remains small and thin. The 

 cerebellum of birds presents many resemblances to that of the 

 Pterosaurs, which can be explained as due to the action of 

 similar modes of life working on related materials. The early 

 stages of development, amnion arid allantois, are very similar. 

 Coming to the skeleton, the single occipital condyle, the inter- 

 orbital septum, the limb girdles, the hollow nature of several 

 of the bones and the mesotarsal articulation of the feet, are all 

 characters which appear in some or most of the reptiles of the 

 Sauropsidan branch. In addition, the Jurassic fossil Archaeo- 

 pteryx had gastralia, a prelachrymal and remnants of two 

 temporal vacuities, a long tail with several separate caudal 



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