64 
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 Centrococcyx hengalensis when passing into adult dress is subjected to 
certain colour changes without moulting (see, also, p. 215 of our text). 
The changes in colour of certain corneous or epidermal parts, such as the 
bills and legs of certain Herons {JHcTodicis^ Bubulcus) 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 differences. 
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, Cinnyridae, many Phasaniidae^ many Anatidae^ etc., etc. 
2. Sexes alike. Examples: Pittidae^ Artamidae^ many Ploceidae ^ many 
Alcedinidae and Cuculidae^ most Ardeidae and Laridae^ etc., etc. 
3. Female more highly developed than the male. Examples: Turni- 
cidae, Phalaropus , Limosa, Hydralector, Centrococcyx, Rhynchaea, Eudromias, 
Casuarius, Dromaeus, the Crypturi and others. 
To these may be added doubtfully: 
?4. Sexes developed on independent lines of evolution. Eudynamls, 
Monachalcyon, Cittura. 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, Eiidynamis melanorhyncha , resembles another 
Cuckoo, Centrococcyx hengalensis, when the latter is in first plumage; Monachalcyon 
monachus, especially the female, bears resemblance to Halcyon hombroni of the 
Philippines; the female of Cittura cyanota is more like both sexes of C. sangirensis 
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 Tanygnathus 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 Phalaropus in which the 
female becomes rather the finer bird, and so on to Turnix in which she is much 
superior to her partner. It also happens at times that the male is the more 
advanced in one resjDect 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. 
