SEXUAL SELECTION 63 
In the first place, we have to notice the presence of 
special weapons, such as horns or tusks, developed 
exclusively or to a special extent in the males of 
those species in which it is the habit of the members 
of this sex to strive together for the possession of the 
females. In such cases the stronger and better-armed 
males are supposed to survive, and to leave a greater 
number of offspring than their weaker rivals; so that 
this form of competition is regarded as acting in 
quite a similar way to natural selection. 
In a second set of cases, of which many remarkable 
instances are to be seen among birds, the males are 
found to exhibit brilliant and varied colours, or to 
possess special decorations in the form of plumes or 
other appendages, or to be gifted with the power of 
song. It is to cases such as these that the term sexual 
selection more properly applies, since the females are 
supposed to bestow their favours upon the most 
beautiful males, and to reject the advances of those 
among their suitors which are less lavishly provided 
with ornament. , 
In these cases, where the development of brilliant 
colours or other ornamental arrangements is believed 
to have taken place owing to the choice of the females 
—particularly in such a case as is represented by the 
peacock’s tail or the wings of the Argus pheasant— 
the supposed change must have come about in direct 
opposition to the action of natural selection, since the 
latter would favour a production of colours resembling 
those of the natural environment for the sake of con- 
cealment, and would hinder the formation of such 
