We will now end this digression on the habits of birds and apply the evidence which we have gathered to the Pelopæus. Every species practising its industry in our dwellings must first have practised and, we maintain, must still practise it under conditions wholly extraneous to the work of man. The Wall-swallow and the Sparrow have given us proofs which are all that can be desired; the Domestic Swallow, more reticent of his secrets, gave us only probabilities, which however come very near to certainty. The Pelopæus is almost as obstinate as the last-named in refusing to divulge her ancient customs and long remained to me an insoluble problem in so far as her original domicile was concerned. Where can the enthusiastic colonist of our chimneys have lived, when far removed from man? Thirty years and more elapsed after I first made her acquaintance; and her history always ended in a note of interrogation. Outside our houses, never a trace of a Pelopæus-nest. And all the time I was applying the method of analogy, which provides a very probable answer to the question of the Domestic Swallow; I was pursuing my search in the caves, in the shelters under rocks facing the sun. Not a sign. I was still continuing my useless investigations, when at last chance, which favours the persevering, thrice compensated me, under conditions which I did not for a moment suspect of being auspicious. The Sérignan quarries are rich in accumulations of broken stones, refuse that has lain piled up there for centuries. These stone-heaps are the refuge of the Field-mouse, who, on a mattress of dried grass, crunches the almonds, olive-stones and acorns which he picks up all around and varies this farinaceous diet with Snails, whose empty shells lie packed under some flat stone. Different Bees and Wasps—Osmiæ, Anthidia, Odyneri—pick out shells to suit them from the heap and build their cells in the spiral. My search for these treasures makes me turn over a few cubic yards of broken stones every year. Three times, when engaged upon this task, I came upon the Pelopæus’ work. Two nests were placed deep down in the heap, against blocks hardly bigger than a man’s two fists; the third was fixed to the lower surface of a large flat stone, forming a canopy above the ground. These three nests, though subject to all the changes of the weather, contained nothing more than the usual structure found inside our houses. The material was plastic mud, as always; the protection, a covering of the same mud; and that was all. The dangers of the site had suggested no improvement to the architect; the edifice was no different from those built against the wall of a chimney. One point is established, therefore: in my district, the Pelopæus nidifies sometimes, but very rarely, in stone-heaps and under natural flagstones which do not touch the ground. Thus must she have nidified before becoming the inmate of our dwellings and our fireplaces.