CHAPTER XXVI 



CONVERGENT EVOLUTION AND PARALLEL DEVELOPMENT 



The evolution of the Owls and diurnal birds of prey. Night-jars and Owls. 

 Swifts and Swallows. The Cariama, Cranes and Storks. The Plover tribe and 

 the Rails. Gulls and Petrels. The Diving Petrels and the Auks. The remark- 

 able transformation of the skeleton in the Auks and Diving Petrels. Homo- 

 plasy a factor in the evolution of birds. The problem of the New World Vul- 

 tures. 



LET US now proceed to examine in some detail a few of 

 the more striking instances of "convergence" as it 

 occurs among birds. 



Perhaps the most striking instance is that afforded by the 

 birds of prey, which may be described under the general term 

 of Hawks and Owls. 



Until comparatively recent times these were regarded as 

 intimately related. And since the Hawk tribe hunt by day 

 while the Owls choose rather the twilight hours for their 

 marauding expeditions, they were divided by the older 

 Ornithologists into two groups — the Diurnal and Nocturnal 

 birds of prey. To-day no one conversant with Ornithology 

 would agree to regard such an association as an expression 

 of genetic relationship. The more or less nocturnal Owls are 

 now invariably recognised as near allies of the crepuscular 

 Night-jars, and descended in consequence from the great Coracii- 

 form stem of the avian tree. The Hawks, Eagles, Falcons 

 and so on, on the other hand, are undoubtedly allied to the 

 Pelargiform or Stork-like birds. 



The common resemblances which the two groups share are 

 due entirely to adaptation to the requirements of a raptorial 

 life, and nowhere is this fact more marked than in the skeleton. 

 Strange though it may appear, the bony framework is more 

 plastic, yields more completely to the moulding influences of 

 the environment, than do the muscular and visceral systems. 



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