CHAPTER XVI 



THE LIFE-HISTORY OF BIRDS— AN CECOLOGICAL SUMMARY 



Embryology in outline. No hard-and-fast line between embryonic and 

 post-embryonic characters. Plumage phases. Some puzzling facts with regard 

 to adult characters. The plumage of Thrushes. Signs of maturity. Nuptial 

 liveries. Coloration and its evolution. Moulting. Abrasion phenomena. The 

 age of birds. Mortality. Play of birds. 



IT is proposed in this chapter to give a general summary 

 of the cecological side of bird-life before proceeding to 

 discuss the aetiological — before we attempt to consider 

 the problem of the origin and causes of the evolutionary data 

 so far dealt with. 



In tracing the life-history of birds it is the common practice 

 to take up only the post-embryonic portion thereof. But 

 there is no hard-and-fast line to be drawn between embryonic 

 and post-embryonic life, as will be shown presently : though 

 it will be sufficient for the purposes of the present book to 

 give but the barest outline of the embryonic stages ; only such, 

 in short, as bear upon post-embryonic life will be considered. 



Briefly, the later stages of development reveal, not seldom, 

 indications of earlier, more primitive structural characters. That 

 is to say, passing phases in embryonic life often represent con- 

 ditions which obtained during adult life in more remote an- 

 cestors. In other words the phylogeny is repeated in the 

 ontogeny — more or less. 



Thus in embryology we have, as it were, the mirror of the 

 past, in which we may see in more or less vague procession the 

 several phases through which any given animal has passed in 

 the course of its evolution, while if we follow the same animal 

 through its post-embryonic life we may witness the acquisition 

 of new characters in this life-history, now at one stage now at 

 another. Thus it has been said that every animal during its 



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