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 Screech 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 

 Linnaeus, the great Swedish botanist, and embodied in his Systema Naturae, 

 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- 



