PEATE RSYOE PHACOCK 
is as a rule the rectrices that are elongated. The plu- 
mage of “ the gaudy Indian bird” is clearly its most 
striking peculiarity. It serves as an excellent instance 
to illustrate the way in which the colours of birds’ 
feathers are sometimes produced. It will be noticed if 
a peacock’s feather be held out and gradually moved 
round from one hand to the other that the shades change 
in hue. It will be furthermore noticed, that if the 
feather be rudely bruised the beautiful iridescent tints 
are lost. Finally, an examination of one of the green or 
blue feathers under the microscope shows merely a 
dingy brownish or black coloration. The gorgeous 
colouring disappears like that of “ the purple jar.” The 
explanation of these facts is that the metallic colours of 
the peacock’s feathers are what have been termed 
“ optical”’ colours. They are not, that is to say, due 
to the presence in the feather of pigments of a green or 
blue hue, but upon a pigment of black the various 
sculpturing of the outer part of the feather produces the 
greens and blues, just as a slice of mother-of-pearl shows 
different hues not present in the shell in the shape of 
pigment. There are, of course, many colours found 
among birds which are due to the existence in the 
feathers of corresponding pigments. Thus the red of 
the touracou, of which we speak on another page, is a 
definite red pigment. Both the common peacock and a 
variety or different species as some think it, viz. Pavo 
migripennis, are to be seen at the Zoo. ‘The interest of 
the latter bird is largely that it suddenly appeared as a 
distinct form, and since then has “ bred true.” It 
seems to be a genuine example of evolution by leaps and 
bounds! The pheasant tribe, to which the peacock, as 
already stated, belongs, is an Asiatic race mainly found 
in the tropical parts of that continent. The Argus 
pheasant, with its enormous train of eyed feathers, is 
perhaps the nearest ally of the peacock among the 
BC 209 P 
