xii A HISTORY OF BIRDS 



CHAPTER XVIII 



PAGE 



Acquired Characters— the Problem of Parental Modifications 311 

 The remarkable case of the Hoatzin and the bearing thereof on the 

 problem of parental modification. The results of feeding experi- 

 ments, and changes of light and temperature on Flamingoes and 

 Tanagers. 



CHAPTER XIX 

 Natural Selection as Applied to Birds 3^° 



The theory of " Natural Selection " applied to birds. Modes of selection. 

 Inter-specific selection. Pigs and Penguins. Skuas and natural 

 selection. Protective coloration. Winter whitening of Ptarmigan. 

 Mimicry among birds. Protective resemblance and aggressive re- 

 semblance. Importance of inter-specific selection on the evolution 

 of species. Intra-selection. 



CHAPTER XX 



Artificial Selection 335 



The evolution of the domesticated races of Pigeons, Fowls, etc. Rever- 

 sion of domesticated races to the wild, ancestral type. Physiological 

 and morphological species. FertiUty and the test of species. 



CHAPTER XXI 



Sexual Selection 343 



Darwin and the theory of sexual selection. The evidence on which his 

 theory was based. Objections to the theory of Alfred Russel 

 Wallace, and of H. Eliot Howard. Sexual selection in absence of 

 secondary sexual characters. Consciousness in display of the effect 

 produced, or to be produced. The factors which incite display. The 

 part which sexual selection does play. 



CHAPTER XXII 



Isolation — Spatial and Physiological and Determinate Evolution - 354 



Isolation and natural selection in evolution. Natural selection and the 

 origin of species. Isolation and geographical distribution. Varia- 

 tions blastogenic. Romanes and Isolation. Physiological selection. 

 Inter-breeding species. Island forms. Species formation in high 

 altitudes. Discriminate isolation. Determinate evolution. 



CHAPTER XXIII 

 Structural and Functional Adaptations — Shape and Symmetry 366 



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. 



