ORIGIN AND CHARACTERISTICS 23 



dark in colour. Less familiar forms of Hetero- 

 chrosis are Xanthochroism and Erythrism, in 

 which an abnormal amount of yellow or red 

 pigment is present. In some cases at least 

 (especially of the latter form) this is the result of 

 exceptional food. Lastly, the arrangement and 

 pattern of colour upon feathers demands a few 

 words of explanation. These to a great extent 

 are due to age, sex, or season. In many species 

 spots and streaks are a sure indication of youth 

 or immaturity, just as transverse bars, on the 

 other hand, are characteristic of the adult stage 

 of existence. In some highly specialised groups 

 (as for instance the Crows) there is but little 

 difference in colour between the nestling and its 

 parent, but in other groups (as for instance the 

 Gulls) several years may elapse before the young 

 finally get rid of their characteristic dress of 

 youth and assume the very different tints of the 

 adult, during this period between youth and 

 maturity practically recapitulating the ancestral 

 changes the species has undergone, it may be 

 from a mottled or spotted form to unsullied grey 

 and white, as in the Gulls and Terns. In a great 

 many birds the male is the most brilliantly 

 coloured, and remarkable for those wonder- 

 ful modifications of plumage that are classed 



