Wallace’s Theory 
the powers of defence and concealment possessed 
by the species.” 
In support of his contention, Wallace asserts 
that all species of birds, of which the hens are as 
conspicuously coloured as the cocks, nest in holes 
or build domed nests. The plumes and other 
ornaments, which the cocks of certain species 
display, Wallace would attribute to a surplus of 
streagth, vitality, and growth power, which is 
able to expend itself in this way without injury. 
“Tf,” he writes, ‘‘we have found a vera causa 
-for the origin of ornamental appendages of birds 
and other animals in a surplus of vital energy, 
leading to abnormal growths in those parts of 
the integument where muscular and nervous 
action are greatest, the continuous development 
of these appendages will result from the ordinary 
action of natural selection in preserving the most 
‘healthy and vigorous individuals, and the still 
further selective agency of sexual struggle in 
giving to the very strongest and most energetic 
the parentage of the next generation.” (Dar 
winism, p. 293.) “Why,” he says, “in allied 
species the development of accessory plumes 
has taken different forms we are unable to say, 
except that it may be due to that individual 
variability which has served as the starting point 
for so much of what seems to us strange in form, 
or fantastic in colour, both in the animal and 
vegetable world.” 
323 
