f 
Albinism, or unusual whiteness, is the sporadic occurrence of white 
individuals, in species that are normally otherwise coloured, and may occur 
in almost any species. It may be either perfect or partial and is due to 
lack of pigment or colouring matter in the feather or skin substance. It 
can be regarded as a manifestation of physical weakness and is said to be 
induced, among other things, by close inbreeding. A good test as to 
whether a pure white coloration is albinism or is normal is the colour of 
the pupils of the eyes. In albinos the pupils are pink in colour, as 
the lack of normal colour in the retina allows the blood coloration 
to show. 
Melanism, or unusual blackness, is due to an excess of pigment. A 
good example of melanism is the Black Fox, which is a melanic variant 
of the common red species. 
Dichromatism is the term applied to occurrence of two different types 
of coloration in a single species, irrespective of sex, age, or season. Thus 
the Sereech Owl occurs in both red and grey phases (see Plate XVI A). 
They breed together indiscriminately and the offspring may be of either 
coloration. The Rough-legged Hawk and the Jaegers occur in light and 
in almost black phases; the difference between dichromatism, and melanism 
in these cases is slight. 
Hybrids form another departure from specific type. They are the 
offspring of parents of two distinct species. It is only occasionally that 
such matings are fruitful, and when they are the offspring is generally 
sterile.. Hybrids occur most often among ducks, especially with the 
Mallard as one of the parents. 
Every North American bird has a common or vernacular name author- 
ized by usage and recognized by the leading ornithologists and there is seldom 
necessity for using the scientific nomenclature. However, it is well for 
all who are interested in birds to familiarize themselves with as many of 
the scientific names as possible, as they are not only necessary in more 
advanced work, but they are of practical use in grasping the general re- 
lationships between various species. 
The present Binomial System of nomenclature was introduced by 
Linneus, the great Swedish botanist, and embodied in his Systema Nature, 
tenth edition, 1758, which is the authority accepted by American ornith- 
ologists. In this system each species is given a double name, the first term 
being that of the genus to which it belongs, the second that of the species. 
Generic names are not duplicated within the sphere of zoology and specific ' 
names never within the genus. Thus, the American Robin is Planesticus 
migratorius; that is, that species of the genus Planesticus which is named 
migratorius. Other species of Planesticus have other names than migra- 
torius. 
The three objects of scientific nomenclature are exactitude, univer- 
sality, and permanence. To this end the naming of zoological material 
is subject to strict laws whose principles are universally accepted and 
applied according to strict codes. Under these laws the scientific name of 
a species is not a matter of personal preference, but is fixed, so that few or 
none can dispute it, and no changes can be made in scientific nomenclature 
except such as are necessary to correct current mistakes in the application 
of the laws of the code. With increased knowledge it has become neces- 
sary to depart slightly in letter, though not in spirit, from the strict bi- 
