The subject of the moon's development has been dealt with by Professor G. H. Darwin by means of a highly abstruse mathematical analysis, which the present writer cannot pretend to be able to discuss. He wishes to point out, however, that Professor Darwin's theory requires the assumption that earth and moon formed, at one time, a single highly-heated fluid mass; the theory being that the moon was thrown off by centrifugal force aided by the sun's tidal influence and synchronous vibratory motion of the fluid mass.