132 MORPHOLOGICAL TYPES 



muscle of the iris. At the same time, the contraction of the 

 ciliary muscle pulls the hinder part of the eye forwards, and 

 this reduces the tension on the suspensory ligaments, which 

 are attached to the lens. 



In the ear, the cochlear part of the saccule is better developed 

 than in reptiles, and is beginning to show the spiral winding. 



With the exception of the warm-bloodedness, and the 

 complete subdivision of the ventricle of the heart, the characters 

 which birds show, and which are not yet developed in the 

 reptiles, are specialisations which do not appear in the mammals. 

 Birds represent a further development of reptiles in one direc- 

 tion, while the mammals evolved in another direction from 

 another group of primitive reptiles. 



Characters of Columba which show an advance on the con- 

 ditions in lower forms {and which are at the same time specialisa- 

 tions not found in mammals) : 



Feathers ; 



Modification of the pectoral limbs into wings ; 



Loss of teeth ; 



Formation of air-sacs (foreshadowed in the 



Chamaeleon) ; 

 Formation of oblique and post-hepatic septa ; 

 Loss of right ovary and oviduct ; 

 Very long sacrum ; and fusion of vertebrae ; 

 Extreme posterior position of pubis ; 

 Loss of left systemic arch. 



Literature 



Parker, T. J. A Course of Instruction in Zootomy (Vertebrate). 

 Macmillan, London, 1884. 



