nest, and, moreover, as said before, something much resembling a Peewit's nest is by such movements actually made. Taking all this together, we have here, as it seems to me, an indication of some such origin of nest-building as that which I have imagined.
As this theory supposes some relation between the nest and the place where pairing takes place—that the one in fact gradually becomes the other—it would be interesting to ascertain whether birds that make their nests in a place which is out of character with their ordinary habits, pair here or amidst their more usual surroundings. For instance, if the Nightjar, a most aërial and arboreal species, were nevertheless to pair habitually upon the ground, this would be a somewhat striking fact. I cannot affirm that it does so. Nevertheless, it is my impression that upon one occasion—which I have recorded in a former paper—I but just missed seeing the pairing of two that I was watching upon the ground and in the near vicinity of the nest. Since then I have seen one pursue another in an obviously amorous or "nuptial" flight from the top of a tree to the ground where it (the pursued bird) settled. The nocturnal habits of this species are, however, a great difficulty in the way of observations of this kind.
The male Wheatear indulges, during the breeding season, in very extraordinary movements of a more or less frenzied nature, and, in watching these, one cannot but be struck by the predilection which seems shown for some natural hollow in the ground, within or over which such movements take place. I have given elsewhere[1] a full account of these actions as exhibited by two rival birds for the greater part of an afternoon, and I will only quote here a few lines which give that incident of the bits of grass which I have already alluded to. I have, it is true, suggested a symbolical explanation, but, however that may be—nor does, perhaps, the one supposition preclude the other—I think what I witnessed shows that a bird may seize something and bring it to a certain spot whilst in a state of violent nervous excitement, and when the intention of building a nest seems pretty well excluded as a cause of such action.[2] If this be so, then, at least, some part of the difficulty which we might feel in supposing a process now become so elaborate, and (in some cases
