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 (Axzthro- 
potdes 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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