This was a natural incubator, certainly, but superior to those in common use in that the solar heat stored by the stone during the day rendered superfluous any large accumulation of vegetable matter. Surely it is but a short and easy step from the perception of solar heat to the conception that such heat would assist in the incubation of eggs. None but a mound-builder who, of course, must have general knowledge on the subject of temperatures and the maintenance thereof, could conceive that these heated rocks would obviate the labour of raking together a mass of rubbish. Further, her inherent perception that moist heat due to the fermentation was vital towards the fulfilment of her hopes of posterity would avert the blunder of trusting to the dry rocks alone. The hot rocks and a small quantity of decaying leaves stood in her case for a huge mound, innocent of extraneous heat. Having, therefore more time to scratch for her living, she would naturally become a more robust bird, more attractive to the males, and the better qualified to transmit her exceptional mental qualities to her more numerous offspring. These are the bare facts. Let those who believe that birds are capable of taking the step from the fact to the principle continue the trains of thought into which they inevitably lead. Will this particular scrub fowl by force of her accidental discovery start a revolutionary change in the life-history of mound-builders generally? Or will the bird----? But there are the facts to conjure or to play with. CHAPTER XX SWIFTS AND EAGLES I. A RARE NEST Among the resident birds one of the most interesting from an ornithological standpoint is that known as the grey-rumped swiftlet (COLLOCALIA FRANCICA), referred to by Macgillivray as "a swallow which Mr. Gould informs me is also an Indian species." That ardent naturalist is, therefore, entitled to the credit of discovery. Sixty-one years had passed since Macgillivray's visit, during which no knowledge of the life-history of the bird which spends most of its time hawking for insects in sunshine and shower had been revealed, when a fragment of a nest adhering to the roof of a cave on one of the highest points of the Island attracted attention. Submitted to an expert (Mr. A. J. Campbell, of Melbourne, Victoria), the identity of the builder was guessed. Subsequently I had the satisfaction of finding a colony close to the water's edge, on the weather side, where the birds had frequently been seen darting among blocks of granite almost obscured by jungle.