Occasional Notes. 141 
more remarkable ingredients of its original pigment — as 
for instance, by withholding food containing metallic 
elements, where these are not commonly distributed in 
food materials generally. 
Through this dependence of coloration on the pres- 
ence of certain kinds of food material from which spe- 
cial elements are extracted, we have a natural ex- 
planation of the variability or of the apparent tendency 
to variability, as regards feather coloration, in birds 
of the same, or closely allied, species under different 
life conditions or in different localities. Probably in 
this same direction must be sought the explanation 
of the changes of coloration in many birds on reach- 
ing their adult stage. A difficulty meets us, however, in 
the case of those male and female birds of the same 
species, where the two sexes take on a strikingly differ- 
ent coloration : for either the male bird eats some special 
kind of food not eaten by the female, and thus derives 
some essential ingredient, not obtained by the female, 
for special pigments ; or there must be an entirely 
different re-arrangement of the essential food elements 
in the two sexes when the food materials are the same, 
and the charafteristic coloration different. The case, 
however, is one that could certainly be determined 
by experiments. Probably also the remarkable diversity 
in the coloration of the same species of domestic fowls, is 
capable of explanation on the lines of food material — 
the diversity being brought about by the varied and 
varying diet of such omnivorous birds, through a long 
course of time — modified in this, as in all other cases, by 
the inter-breeding of birds of different coloration. 
In the case described by Mr. IM Thurn as practised 
