Subject to change also are the oases; and in the same manner apparently as the canals. They grow less evident at a like season of the Martian year. They do this seemingly by decreasing in size. Whereas in the full expanse of their maturity they show as round spots of appreciable diameter, as the season wanes they contract to the smallest discernible of dots. All but the kernel, as it were, fades out, and even this may disappear from sight. The Phoenix Lake in its summer time is a very dark circular spot, small indeed yet of definite extension; in its winter it shrinks to a pin point, and is often not visible at all. Sometimes the husk apparently persists, a ghostlike reminiscence of what it was, with the kernel showing dark-pointed in its centre. Thus the Lucus Lunae appeared at the opposition of 1905. A faint wash betokened the presence of the Lucus, through which now and again a black pin-point pierced.

In this visible decrease of size we get a revelation as to what takes place impossible in the case of the canals, the tenuous character of which precludes more than inference as to the process.

Like the canals, latitude, together with the suitable season of the planet’s year, are the determining factors in their development. In what corresponded to our July of the northern hemisphere the oases in the sub-arctic and north temperate zones were conspicuous; black spots that showed in profusion along the parallels of 40°, 50°, and 60° north. At the same time the equatorial ones, those along the Eumenides-Orcus, which had been most evident in 1894, hardly came out. It had been their time then as it was that of the others now. The law of development is not so simple as on the earth, depending, like that of the canals, not only upon the return of the sun, but upon the advent of the water let loose from about the polar caps. Thus the equatorial oases are subject to two seasonal quickenings, one from the north, the other from the south.

In regard to their method of evolution or devolution a most curious observation happened to me in 1903. Usually the oases are of solid tone throughout; equally sombre from centre to circumference. But in this case such uniform complexion found exception. On March 1, 1903, the Ascraeus Lucus came out strangely differentiated, a dark rim inclosing a less dark kernel. The sight was odd enough to command comment in the shape of a sketch which accompanied the note, and the further remark that other spots had similarly that year affected the like look. That the effect was optical did not seem to me the case. Other spots at other times showed nothing of the sort. If it was due to objective cause it gathers circumstance from what was then the Martian time of year. For the season was such that the spot should then have been in process of waning; and the effect would indicate that in so doing the tone of the centre went first, that of the circumference fading last. This would be in accordance with a growth proceeding outward and a decay that followed in its steps.

When to this we add the look of the oases at the antithetic season,—often a faint shading only, with or without a darker pin-point at its core,—we are led to the belief that the area of the oasis is unchangeable and that its growth means a deepening of tint.

So far, then, as it is possible to particularize them, the oases develop from a small nucleus, perhaps twenty miles in diameter, perhaps less, and from this spread radially till they attain a width of seventy-five or one hundred miles. If the oasis be associated with a double canal, this maximum width exactly fits the space between the twin lines. Even when no double enters the oasis, the size is about the same. This size attained, they hold it for some months. Then they proceed to fade out to their initial nucleus, and after a sufficient rest the process starts over again.

With the carets something of the same sort seems to take place—if we may consider as betokening a general law the fact that in 1894 the carets at the mouths of the Phison and Euphrates developed before their affiliated canals. But about them much less is yet known, and we must be content to say that the observations of 1905 made at the opposite season of the canal’s year seem to bear this out.