2i6 The Naturalist in La Plata.
Gould tried to obtain others, offering as much as fifty pounds for one; but no second specimen ever gladdened his eyes, nor was anything more heard of it until Stolzmann refound it in the year 1880.
The addition of many new species to the long list would, however, be a matter of small interest, unless fresh facts concerning their habits and structure were at the same time brought to light; but we can scarcely expect that the as yet unknown species will supply any link connecting the Trochilidae with other existing families of birds. The eventual conclusion will perhaps be that this family has come down independently from an exceedingly remote past, and with scarcely any modification. While within certain very narrow limits humming-birds vary more than other families, outside of these limits they appear relatively stationary; and, conversely, other birds exhibit least variability in the one direction in which humming-birds vary excessively. On account of a trivial difference in habit they have sometimes been separated in two sub-families: the Phaethornithinae, found in shady tropical forests; and the Trochilinae, comprising humming-birds which inhabit open sunny places--and to this division they mostly belong. In both of these purely arbitrary groups, however, the aerial habits and manner of feeding poised in the air are identical, although the birds living in shady forests, where flowers are scarce, obtain their food principally from the under surfaces of leaves. In their procreant habits the uniformity is also very great. In all cases the nest is small, deep, cup-shaped, or conical, composed of soft felted mate-
Humming-Birds. 2 17
rials, and lined inside with vegetable down. The eggs are white, and never exceed two in number. Broadly speaking, they resemble each other as closely in habits as in structure; the greatest differences in habit in the most widely separated genera being no greater than may be found in two wrens or sparrows of the same genus.
This persistence of character in humming-birds, both as regards structure and habit, seems the more remarkable when we consider their very wide distribution over a continent so varied in its conditions, and where they range from the lowest levels to the limit of perpetual snow on the Andes, and from the tropics to the wintry Magellanic district; also that a majority of genera inhabit very circumscribed areas--these facts, as Dr. Wallace remarks, clearly pointing to a very high antiquity.
It is perhaps a law of nature that when a species (or group) fits itself to a place not previously occupied, and in which it is subject to no opposition from beings of its own class, or where it attains so great a perfection as to be able easily to overcome all opposition, the character eventually loses its original plasticity, or tendency to vary, since improvement in such a case would be superfluous, and becomes, so to speak, crystallized in that form which continues thereafter unaltered. It is, at any rate, clear that while all other birds rub together in the struggle for existence, the humming-bird, owing to its aerial life and peculiar manner of seeking its food, is absolutely untouched by this kind of warfare, and is accordingly as far removed