4. VARIATION. 



The phases of Variation, or Modification of Structure and Plumage displayed 

 among the birds of Celebes may be conveniently considered under the following 

 headings : 



1. Individual Variation; the differences peculiar to the individual. 



2. Geographical Variation; as shown by local races, subspecies, or species. 



3. Seasonal Changes; such as peculiar summer and winter plumages in birds. 



4. Sexual Differences; the secondary sexual characters. 



5. Changes depending upon Age; the development and decadence of the 

 individual. 



1. Individual Variation. 



The assumption that no two individuals are ever exactly alike seems to 

 be completely justified by facts. No one, probably, is so fully aware of this, 

 as the zoologist, who is called upon to make the closest possible comparison 

 of large series of individuals of the same race. In the course of writing the 

 present book, for instance, which is chiefiy based upon a study of the ex- 

 ternal coverings, and bills, legs and feet of Celebesian birds, with occasional 

 reference to their skeletons — some thousands of specimens have been examined, 

 yet to the best of our knowledge no two of them were exactly alike; moreover 

 in the text several thousand measurements of parts will be found, yet we believe 

 that hardly any two cases occur in which four terms (wing, tail, tarsus, billi 

 are the same. There are some very close observers of Nature to whom a know- 

 ledge of this infinite diversity of form is perceptible; who, as children, are 

 conscious of the peculiarities of individual Sparrows or the differences in blades 

 of grass ; others, and amongst them men of learning, have never had their eyes 

 opened to the fact, and assert that exactly the opposite is the case. Thus the idea 

 of a uniformity of the individuals of a species is encouraged by the latter, with 

 its consequence, that species were evolved per saltum. This position is partly 

 the result of a system of nomenclature which no longer meets the needs of our 

 time. There are of course species — groups of individuals possessing some 

 character (or characters) never so found in any other group, — but each indi- 

 vidual has its own peculiarities, and an ideal system of nomenclature can only 



