Greater Value of Females 
tion will cause the species to become extinct, 
by the elimination of all the males. Or, let us 
suppose that the mutation in the direction of 
showy plumage affects both sexes, then in such 
a case the species will almost certainly become 
extinct. If, however, the hypothetical species 
nested in holes in trees, it is quite possible that 
it might survive notwithstanding its showy 
plumage. 
Whether, as Wallace suggests, the hen does 
most of the incubating, and is exposed to special 
danger when sitting on her eggs in an open nest, 
or, as Stolzmann urges, it is of advantage to the 
species that there should not be too many males, 
the result is the same, that the species can afford 
to allow the cock to be more gaily attired than 
the hen. In either case the colouration of the 
cock becomes a matter of comparatively little 
importance to the species, and this, coupled with 
the fact that the male tends to mutate more 
readily than the female, will explain why, in 
most species which exhibit sexual dimorphism, 
it is the cocks that are the more conspicuous. 
In certain species the cocks alone incubate, and 
these then become more important than the 
females to the race, so that they have not been 
permitted to become showy, while the hens have 
been allowed more freedom in this respect. 
The extreme variability of the Ruff (Pavon- 
cella pugnax) in breeding plumage points to the 
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