290 THE ZOOLOGIST. 
in questions of the relationships of birds. The difference in 
colour between the extreme S. menzbiert and S. vulgaris seems 
enormous, yet when we inquire into the source of these tints we 
learn that the only difference lies in the thickness of the trans- 
parent outer layer on certain parts of certain feathers, and at 
the outside this difference cannot be more than one fifty- 
thousandth part of an inch, and is probably much less. 
In certain birds these metallic feathers are variable to some 
extent. I recollect once noting this in examining the purple 
specula of a batch of male Pintails. Often we find metallic 
colours appearing more or less sporadically in certain birds—for 
example, Ruff, Common Snipe, or even Red Grouse. This is 
easily explained by the theory of thin plate colours, which 
depend upon the simplest possible structures, but the pheno- 
menon would be inexplicable to one who believed that metallic 
feathers owed their lustre to an elaborate system of prisms. 
It is not then surprising to learn that Dr. Sharpe failed to 
reduce the chaos of our forms of Starlings by a study of their 
colours. Really one is justified in discussing the possibility of 
S. menzbiert leaving Siberia in its typical form, and arriving in 
England as the green-headed S. vulgaris merely through the 
wear and tear of the journey. Fatio, working along quite 
different lines, attempts to show how this is possible, and with 
the Starling ; and I have myself tried the plan of rubbing the 
feathers with a pad of blotting-paper, in an attempt to wear 
down the colour. ‘lhe best way is to cover half the feather with 
a piece of thin tough paper, so that the worn part may be com- 
pared later with the protected part. There is certainly a change 
of colour, and one in the right direction—-from purple to green, 
but I must confess that I cannot hold the experiment to be free 
from the chances of error, nor productive of unequivocal results, 
owing to the fact that the feather presents not a flat surface, but 
a series of tiny curved facets. 
The longer we live the more conscious we are of our ignorance 
of the real meanings of bird coloration. Mere questions of 
values, as discussed by that sagacious and keen-witted American, 
Mr. A. H. Thayer, may be solved, as he has worked, by accurate 
observation ; but when we come to colour, we are at sea immedi- 
ately. Years ago Dr. Waelchili demonstrated that some birds 
