4. VARIATION. 
The phases of V ariation, 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 chiefly 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, bill) 
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 
