"JQ Introduction: Variation. 



5. Changes depending upon Age. 



The modifications of ])himage and structure displayed during the life-time 

 of the individual, the phenomena oi' 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 fDescent of 

 Man, p. 187) gives six "classes of cases or rules nnder 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. 1S93, p. 213) 

 adds two classes more. All eight of them have representatives among Celebesian 

 birds, and they allow of rc-grouping according to the phase of variation which 

 exerts a predominant influence in each case. 



Sexual influences predominate in four classes: 



1 . Male more highly developed than female : young like female 



[Loriculus, Cinni/ris, etc.). 



2. Female more highly developed than male: young like male Turnix). 



3. Male like female: young like the parents (Many Psittaci, 



Columbae, etc.). 



4. Male unlike' female: young male like adult male, young female 



like adult female [Monachalcj/on, Cittiira)^). 



The influence of seasonal variation appears to be prepotent for 

 the following: 



5. Male like female: young like the adults in winter plumage Bubulcus), 



or like them in summer plumage {Alca'^, 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: 

 t). Male like female: young different from both Mitnia, Lams, Ardea, etc.). 



7. Male unlike female: young different from both Siphia, Chalcophaps, 



]£iid^)iamis,'^) etc.). 



8. Male unlike female: young ones different, and differing sexually 



from one another [Graucahis bicolor). 



1) Probably a higlier development: see antea, p. CJ. 



-) The condition — male unlike female: young male like female, young female like male — is not known. 

 ') In Eudynaiiiis the culoration of the young is supposed to be protective (see Whitehead, Ibis 1888, 

 p. 410; and Expl. Kina Balu iy.)3, p. 145 . 



