^j^ Introduction: "Variation. 



general among birds. The difficulties of making observations are great, and no 

 certain evidence has been adduced from Celebesian birds, but Prof. W. Blasius 

 holds that Centrococct/.v bengalensis when passing into adult dress is subjected to 

 certain colour changes without moulting (see, also, p. 215 of our text). 



The changes in coloiu- of certain corneous or epidermal parts, such as the 

 bills and legs of certain Herons [Herodias, Bubiikus in the breeding and winter 

 seasons, may perhaps be placed in the same category as changes of colour in 

 the feathers without a moult (see p. 838). 



4. Sexual diflferences. 



In relation to sex it is convenient to gather birds into three groups: 



1. Male more highly developed than the female. Examples: Para- 

 diseidae, Trochilidae, Cinni/ridae, many Phasamidae, many Anatidae, etc.. etc. 



2. Sexes alike. Examples: Pittidae, Artamidae, many Ploceidae, many 

 Alcedinidae and Cuadidae, most Ardeidae and Laridae, etc., etc. 



3. Female more highly developed than the male. Examples: Turni- 

 cidae, P/ndaropus , Limosa, Ili/dralector , Centrococcyx , Phynchaea, Eudromias, 

 Casuarius, Dromaeus, the Crypturi and others. 



To these may be added doubtfully: 



■?4. Sexes developed on independent lines of evolution. Eudyiiamis, 

 Monachahyon, Citturu. The sexes either differ in coloration from the nest or 

 after the first plumage; nevertheless there is some reason to think that the 

 adult female represents an earlier stage in the evolution of the race, and that 

 the species concerned should, therefore, be placed in the first group. Thus the 

 female- of the Black-billed Koel, Eudynamis melanorhyuc/ia , resembles another 

 Cuckoo, Centrococcyx benpah'tisis, when the latter is in first plumage; Monachakyon 

 munachus, especially the female, bears resemblance to Halcyon homhroni of the 

 Philippines; the female of Cittura cyanota is more like both sexes of C. sangireiisis 

 than is the male. 



It is not to be understood that these groups are always sharply characterized 

 and easily distinguishable; on the other hand, gradual transitions from one 

 group to another are found : from such contrasts of the sexes as are seen in 

 Paradisea, Gallus and Cinnyris in which the male far surpasses the female in 

 adornment, to Tavyyuathus and Zosterops in which the female is hardly inferior 

 to the male, to Pitta and Myristicivora in which there is nothing to the human 

 eye to choose between the two sexes, to Limosa and Pkalaropus in which the 

 female becomes rather the finer bird, and so on to Tuniix in which she is much 

 superior to her partner. It also happens at times that the male is the more 

 advanced in one respect and his mate in another; thus, among the Birds-of-prey 

 the male generally has the more highly developed plumage, but the female is 

 of larger size. 



