40 THE BIOLOGY OF BIRDS 



this kind are of high value, for they disclose the primary 

 significance of differences which may subsequently be 

 sifted by some mode of selection. 



Variations in Colouring. — Quite different from modifi- 

 cations imposed more or less from without are variations 

 which arise from within owing to some disturbance in the 

 germ-cells. Thus a true albino without any pigment may 

 arise as a germinal variation ; a hereditary factor has 

 dropped out of the inheritance. Perhaps it is the factor 

 for the pigment-forming substance (the chromogen) ; 

 perhaps it is the factor for the ferment which must operate 

 upon the chromogen before the pigment can be produced. 



Similarly, melanistic variations appear to be of frequent 

 occurrence, where there is a superabundance of dark pig- 

 ment. A darkening of the plumage might of course be an 

 individual modification, but there are cases where the 

 darkening is seen in a large section of a species, and here 

 we have probably to do with the result of germinal variation. 



Albinism. — There are many records of albinism or 

 absence of pigment in birds. It may be total, but it is more 

 frequently partial. When total, it implies that the hereditary 

 factor or factors for pigment production must have dropped 

 out of the inheritance. In some cases, at least, there is a 

 factor for the pigment-producing material, or chromogen, 

 e.g. tyrosin, and a factor for the ferment which activates the 

 chromogen, e.g. tyrosinase. When partial, it may mean a 

 general deficiency in the pigment available or in ferments 

 required to give the pigment-forming material its coloured 

 expression. When very local it may mean nothing more 

 than a disturbance in the normal blood-supply of a certain 

 area of growing feathers. Albinism is not to be confused 

 with normal whiteness, in which different pigment factors 

 neutralise one another in expression. 



Albino blackbirds, crows, sparrows, and swallows are 

 well known. L. Petit has collected a number of rarer 

 cases (1909), e.g. an eared grebe with all the dorsal surface 

 dull white, a cream-coloured (isabelline) woodcock, a 

 partridge of the same colour, a blackbird covered with white 



