CHAPTER XXIII 



STRUCTURAL AND FUNCTIONAL ADAPTATIONS— SHAPE AND 



SYMMETRY 



The factors determining shape. The remarkable asymmetry of Owls' ears. 

 The wing. The shoulder girdle and sternum. The pelvic girdle. The pelvic limb. 

 Some puzzling features in the pelvis of Struthious birds. The pectineal process 

 and its homologies. Types of feet. Feathers and adaptation to arboreal life. 



THE present and succeeding chapters are intimately con- 

 nected with those immediately preceding. Herein it 

 is proposed to discuss in detail the complex problem 

 of adaptation, and thereby the full significance of the facts of 

 variation, acquired characters, selection and isolation, will become 

 apparent. 



Let us begin with the general problem of the shape of the 

 bird's body, for this represents the sum of all its several adap- 

 tations. 



To secure and perfect the power of flight has been the main 

 objective in the determination of this shape ; and to support 

 the body alternately in the air or on the land, intense specialisa- 

 tion has become necessary. The outcome of this has been the 

 conversion of the fore-limbs into the unique organs we call the 

 wings, and the readjustment of the balance of the body so as 

 to permit of its being carried, when on the ground, by the hind- 

 limbs alone, thereby converting the bird into a biped. As a 

 further concession to the demands of aerial locomotion the trunk 

 has assumed a sub-conical shape, though the body, as a whole, 

 during flight is spindle shaped, a form due to the carriage of 

 the head and neck. In certain long-necked swimming birds 

 this spindle shape is much attenuated, the head and neck being 

 carried straight out, as in the Duck tribe, and the Storks and 

 Flamingoes, for example. In other equall}^ long-necked forms, 

 like the Heron — which is also one of the Stork family — the head 

 and neck are brought close to the body, thus materially shorten- 



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