70 
Intro duction : V ariation . 
5. Changes depending upon Age. 
The modifications of plumage and structure displayed during the life-time 
of the individual, the phenomena of its development and decadence, may fitly 
be placed at the end of this chapter, as one form or other of the four prece- 
ding phases of variation — sexual, seasonal, geographical, and (if perpetually 
recurrent) individual variation — is often repeated during the growth of the 
young towards maturity. 
Classification of the developmental phases. — Charles Darwin (Descent of 
Man, p. 187) gives six “classes of cases or rules under which the differences 
and resemblances, between the plumage of the young and the old, of both sexes 
or of one sex alone, maybe grouped”. Keeler (Evol. Col. Feath. 1893, p. 2 I 3) 
adds two classes more. All eight of them have representatives among Celebesian 
birds, and they allow of re-grouping according to the phase of variation which 
exerts a predominant infiuence in each case. 
Sexual influences predominate in four classes: 
1. Male more highly developed than female: young like female 
[Loriculus, Cinnyris., etc.). 
2. Female more highly developed than male: young like male [Turnix). 
3. Male like female: young like the parents (Many Psittaci, 
Columhae., etc.). 
4. Male unlike') female: young male like adult male, young female 
like adult female [Moyiachalcyon^ Citturd)^). 
The influence of seasonal variation appears to be prepotent for 
the following: 
5. Male like female: young like the adults in winter plumage [Bubidcus)., 
or like them in summer plumage (Afca), or intermediate between 
summer and winter plumage [Charadrius) . 
The influence of some previous condition in the history of 
the race (hereditary geographical or individual modi- 
fication) is sometimes satisfactorily, more often doubt- 
fully, displayed under the following conditions: 
6. Male like female: young different from both {M.unia.^Ijarus Ardea^ etc.). 
7. Male unlike female: young different from both {Siphia, Chalcophaps, 
Eudynamisf) etc.). 
8. Male unlike female: young ones different, and differing sexually 
from one another [Grancalus bicolor). 
1) Probably a higher development: see antea, p. 64. 
-) The condition — male nnlike female : young male like female, young female like male — is not known. 
3) In Eudynamis the coloration of the young is supposed to be protective (see Whitehead, Ibis 1888, 
p. 410; and Expl. Kina Balu 1893, p. 145). 
