Page:The Moon (Pickering).djvu/82

56 they transported to ova Earth. Secondly, since there are no high winds upon the Moon, if it were any advantage to plant life to lift itself above the surface of the ground it could do so with safety, instead of clinging close to the rocks, like our own arctic and antartic flora.

My attention was first drawn to the "variable spots," as I then called them, while observing at Arequipa in 1893. On leaving there it was possible to give little attention to the matter until 1901, when a return to a low latitude in Jamaica enabled me to continue the researches under suitable atmospheric conditions. The general phenomena exhibited by a variable spot are a rapid darkening, beginning shortly after sunrise, followed by an equally rapid fading toward sunset. The darkening is sometimes accompanied by a diminution in size, and the fading by an increase. Near sunrise and sunset the spots are almost invisible. At their maximum some of the spots are intensely black, some are a dark gray, and others a light gray. Near the equator the changes in density occur frequently in the course of a few hours after sunrise; in higher latitudes several days pass before the changes begin, but they are then usually very rapid. No spots are known north of latitude +55° or south of latitude—60°. The spots are always associated with small craterlets or deep, narrow clefts, and are often symmetrically arranged around the former. This again suggests volcanic activity and the expulsion of water-vapour. When found inside of a crater, they always, unless very extended, occupy the lowest portion of the floor. If the floor is smooth and level, few changes of interest occur in the spot during the lunation; but if rough, very marked changes are liable to be seen. Since those spots found near the centre of the lunar disk are blackest when the Moon is full and fade out at sunrise and sunset, it is evident that they cannot be due in any way to shadows, which are geometrically impossible at full moon. Consequently, there must be a real change of some sort in the nature of the reflecting surface. Organic life resembling vegetation seems to be the only simple explanation of this change, and if we consider the long lunar day as being analogous, on a small scale, to our terrestrial year, the theory of such life seems to be an adequate explanation—coming up, flourishing, and dying, just as vegetation springs and withers on the Earth. At least, the burden of proof would seem to lie with those who have any other solution of the observed facts to offer.

A good example of a variable spot is found in the crater known as Franklin,. 3A [25. 3.3]. See also Plate D, Figs. 6, 7, 8, 9 and 10. When the Sun first rises upon