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 that only one is really hatched. Yet Belt expressed the opinion that in Nicaragua, where he observed humming-birds, they out-numbered all the other birds together. Considering how abundant birds of all kinds are in that district, and that most of them have a protective colouring and lay several eggs, it would be impossible to accept such a statement unless we believed that humming-birds have, practically, no enemies.