176
THE ESSEX NATURALIST.
ferment which activates the pigment to the failure of the cellular
structure and hypothecate that albinism is a hereditary defect of
structure. They suggest "that these abnormal pigment con-
ditions are not wholly independent and that as a working hypothe-
sis it is reasonable to suppose that complete albinism, partial
albinism, incomplete albinism and xanthism, all static forms of
leucosis, are phases of the same process." We must consider to
what extent this suggested cause of albinism, i.e., a hereditary
defect of structure, may be true of birds. It has already been
shown that avian albinism is inherited in some cases and also
persists, but on the other hand it has been shown that a normally
coloured bird may develop albinism and even return to normality,
that a nestling albino may become normal, and that albinism
may increase during the life of the bird. In spite of these con-
tradictions we know of no explanation of albinism which appeals
to us so strongly as that given. It is not impossible that a
tendency to albinism may be inherited without being exhibited
and be able to be aggravated or reduced under certain
conditions. Although little has been done toward solving the
problems of albinism it has attracted a certain amount of
attention. In 1853 the Rev. A. C. Smith published a list of
fifty-seven species, of which albinistic examples had been
known to occur. It is probable that most if not all species
are subject to this abnormality.
Xanthism is used when speaking of plumages which are
abnormally yellow, that is to say, in cases where yellow has
replaced some other colour or colours. This is said to affect mostly
red or orange feathers. It has been shown by the Hon. Walter
Rothschild that most if not all green birds show yellow varieties.
Xanthism is attributed to weakness of the pigment factor, and if
this is so, and it is highly probable, then it may be nothing more
than a certain stage of albinism in certain colours. It must be
borne in mind that green colour is not due to green pigment, but
to dark melanin pigment and structural interference. As has
been already shown there is a close correlation between pigment
and structural colour modifications. I cannot find that natural
instances of xanthism in red plumage have been described. In
Brazil this phase is produced artificially in some species of Chry-
sotis. This is effected by rubbing the cutaneous secretion of a
toad (Bufo tinctorius) into the budding feathers of the head,