Cranes 



that mutations or discontinuous variations occur 

 in nature ; and as these afford much more favour- 

 able material on which natural selection can act, 

 it is reasonable to suppose that they have played 

 a considerable part in evolution. 



When discussing the phenomena of inheritance, 

 we attempted to show that, not improbably, these 

 discontinuous variations are due to some re- 

 arrangement in the constituent parts of the unit 

 characters, or biological molecules, as we have 

 called them. 



In this connection we may mention the 

 apparently singular phenomenon of different 

 species in the same natural group, exhibiting 

 either a definite excess or deficiency of plumage 

 on the head. Among cranes, most species are 

 more or less bald ; but the Demoiselle (Antkro- 

 poides virgo) has a fully- feathered head with 

 long side-plumes, while the head of the Stanley 

 Crane (A . paradisea) appears to be swollen, so 

 abundantly is it feathered The crowned cranes, 

 although bare - cheeked, have double crests, 

 the two parts of which have been respectively 

 compared to a pen -wiper and a bunch of 

 toothpicks ! 



Among the guinea-fowls, several species are 

 crested, while others, as, for example, the 

 domestic one, are bare-headed. Now, on the 

 theory of evolution, by accumulation of minute 

 variations, phenomena such as these are difficult 



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