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
from all competition with other birds as the solitary savage is removed
from the struggle of life affecting and modifying men in crowded
communities. The lower kind of competition affecting hummingbirds, that
with insects and, within the family, of species with species, has
probably only served to intensify their unique characteristics, and,
perhaps, to lower their intelligence.

Not only are they removed from that indirect struggle for existence
which acts so powerfully on other families, but they are also, by their
habits and the unequalled velocity of their flight, placed out of reach
of that direct war waged on all other small birds by the rapacious
kinds--birds, mammals, and reptiles. One result of this immunity is that
humming-birds are excessively numerous, albeit such slow breeders; for,
as we have seen, they only lay two eggs, and not only so, but the second
egg is often dropped so long after incubation has begun in the first