HETEROCHROSIS IN ESSEX BIRDS AND THEIR EGGS. 175
that albinism does affect what he describes as structural colour
modifications, the significance of which will be made clearer
later. Mr. Chandler examined the feathers of an albino Mallard,
Anas platyrhynchos, to see whether the morphological modi-
fication involved in the production of the violet speculum would
be lost or retained with the lack of pigment. It was found that
the special structure necessary for the production of the colour of
the speculum did not exist in the albinistic bird and that the
feathers had the same structure as those feathers of this species
which are not subject to modification for colour production.
So far as this one example will permit us to say, the factor which
is responsible for the morphological specialisation of the structure
of feathers, and the factor which is responsible for the accom-
panying pigment, are in some way closely related, in other words,
when the pigment is absent so also is the special structure which
is necessary to certain colour effects.
The terminology of albinism requires consideration. What
is a true albino ? This term appears to be used when not only
the feathers but also the other external parts are seemingly
devoid of pigment. There can be no doubt that the expression
has an empirical origin. By some it is used to connote that
complete and partial albinism are not merely a question of
difference of degree, but of nature. The word "true" in this
sense may have a misleading effect and it would be better to
substitute "complete."
Although albinism in birds cannot be considered altogether
along the same lines as in man, for he does not shed his external
covering as birds do, yet the phenomenon is one and the same,
and the work done on man may give us a lead for birds. "A
Monograph of Albinism in Man," by Pearson, Nettleship and
Usher, is a very important contribution to the subject. These
authors say that it is practically impossible to test the complete
absence of pigment from the eye without microscopic examina-
tion of sections of the iris, choroid and retina, so that if we use the
term "Complete albinism" it can only be used to indicate that
the coverings of the bill, legs and eyes as well as the plumage are
affected, and not that there is a complete absence of pigment.
Their views regarding the probable cause and nature of the
phenomenon carry much weight even though they state that
they are hypothetical. They attribute the local absence of the