172 THE ESSEX NATURALIST.
the hereditary factor or factors, which promote the production
of pigment, have dropped out of inheritance. This statement
raises an interesting question, for it would mean that the pheno-
menon cannot develop to completeness during the life of a
bird. I have written to the author and in reply he has kindly
amplified his views along the following lines. He defines total
albinism as complete absence of pigmentation, and he knows
of no case of total albinism that is not present in the newly
hatched bird, or, to put it in other words, he believes total
albinism to be a germinal variation, and that partial albinism on a
diffuse scale arises in the same way, but he considers that slight
albinism, e.g., in part of the wing-feathers, may be an individual
modification, due to some defective blood-supply. The same
author informs us that in some cases, at least, there is a factor for
the pigment-producing material, chromogen, e.g., tyrosin, and a
factor for the ferment which activates the chromogen, namely,
tyrosinase. If this is so then I suppose we may say that before
the pigment-producing matter can do its work it has to be
modified by something in the nature of enzymic action. Thus,
when albinism is partial it may result from a general deficiency
in the available pigment, or in the ferments which bring about
the necessary change therein, or in both. These ideas are of
great interest, but it must be kept in view that they are of a
largely theoretical nature.
Theory cannot be avoided without depriving the subject
of much of its interest and of its chances of progress, but our
main purpose is of a practical nature. The views of Mr. W. P-
Pycraft are of a less theoretical character ; they deal more with
effects than causes, and thus shed light on the difficulties of the
problem which are of a very perplexing nature. We are in-
formed by this writer that only 10% of the albinos recorded
exhibit the phenomenon completely, that is to say, that the iris
and the coverings of the beak and feet retain their normal
colouring in nine out of every ten instances recorded. Mr.
Pycraft makes the definite statement that nothing is known of
the causes which produce white varieties or true albinos. Some
colours are apparently more subject to albinism than others, and
I think that it is the general experience that dark plumaged birds
are more affected than those whose external covering is of a
lighter hue. Red and green plumages, however, are said to be