264 The Naturalist in La Plata. 
select males for their superiority in some esthetic 
quality, such as graceful or fantastic motions, 
melody of voice, brilliancy of colour, or perfection 
of ornaments. Doubtless all birds were originally 
plain-coloured, without ornaments and without 
melody, and it is assumed that so it would always 
have been in many cases but for the action of this 
principle, which, like natural selection, has gone on 
accumulating countless small variations, tending to 
give a ereater lustre to the species in each case, and 
resulting in all that we most admire in the animal 
world—the Rupicola’s flame-coloured mantle, the 
peacock’s crest and starry train, the joyous melody 
of the lark, and the pretty or fantastic dancing 
performances of birds. 
My experience is that mammals and birds, with 
few exceptions—probably there are really no excep- 
tions—possess the habit of indulging frequently in 
more or less regular or set performances, with or 
without sound, or composed of sound exclusively ; 
and that these performances, which in many animals 
are only discordant cries and choruses, and uncouth, 
irregular motions, in the more aérial, graceful, and 
melodious kinds take immeasurably higher, more 
complex, and more beautiful forms. Among the 
mammalians the instinct appears almost universal ; 
but their displays are, as a rule, less admirable 
than those seen in birds. ‘There are some kinds, 
it is true, like the squirrels and monkeys, of arbo- 
real habits, almost birdlike in their restless energy, 
and in the swiftness and certitude of their motions, 
in which the shghtest impulse can be instantly ex- 
pressed in graceful or fantastic action; others, like 
