life have operated with comparatively little check for countless ages, and have resulted in those infinitely varied and beautiful forms--those wonderful eccentricities of structure, of function, and of instinct--that rich variety of colour, and that nicely balanced harmony of relations--which delight and astonish us in the animal productions of all tropical countries. IV. HUMMING-BIRDS: AS ILLUSTRATING THE LUXURIANCE OF TROPICAL NATURE. Structure--Colours and Ornaments--Display of Ornaments by the Male--Descriptive Names--The Motions and Habits of Humming-birds--Food--Nests--Geographical Distribution and Variation--Humming-birds of Juan Fernandez as illustrating Variation and Natural Selection--The relations and affinities of Humming-birds--How to determine doubtful affinities--Resemblances of Swifts and Humming-birds--Differences between Sun-birds and Humming-birds--Conclusion. There are now about ten thousand different kinds of birds known to naturalists, and these are classed in one hundred and thirty families which vary greatly in extent, some containing a single species only, while others comprise many hundreds. The two largest families are those of the warblers, with more than six hundred, and the finches with more than five hundred species, spread over the whole globe; the hawks and the pigeons, also spread over the whole globe, number about three hundred and thirty, and three hundred and sixty species respectively; while the diminutive humming-birds, confined to one hemisphere, consist of about four hundred different species. They are thus, as regards the number of distinct kinds collected in a limited area, the most remarkable of all the families of birds. It may, however, very reasonably be asked, whether the four hundred species of humming-birds above alluded to are really all distinct--as distinct on the average as the ten thousand species of birds are from each other. We reply that they certainly are perfectly distinct species which never intermingle; and their differences do not consist in colour only, but in peculiarities of form, of structure, and of habits; so that they have to be classed in more than a hundred distinct genera or systematic groups of species, these genera being really as unlike each other as stonechats and nightingales, or as partridges and blackcocks. The figures we have quoted, as showing the proportion of birds in general to humming-birds, thus represent real facts; and they teach us that these small and in some respects insignificant birds, constitute an important item in the animal life of the globe.