The artificiality of the canals is shown by the "supernaturally regular appearance of the system, upon three distinct counts: first, the straightness of the lines; second, their individually uniform width; and, third, their systematic radiation from special points." It was the mathematical shape of the Ohio mounds that first suggested their artificial character. That these lines are artificial and not natural is seen in the fact that at times they are not visible. The lines while temporary in appearance are permanently in place. "Not only do they not change in position during one opposition; they seem not to do so from one opposition to another." "Unchangeable, apparently, in position, the canals are otherwise among the most changeable features of the Martian disk." The order of their appearance synchronizes with the changes of the season, as the snow caps begin to melt the canals begin to appear; in appearance strengthened first at the borders of the polar seas and gradually stretching down towards the equator. In minute detail Lowell presents the successive visibility of the different canals. To account for all these phenomena we have to look at our own Earth for a parallel, and we see it in the great irrigation tracks of the West, and in the vast irrigated regions in India depending upon the melting of the Himalaya snow cap. The accumulative evidence is overwhelming that here is a dry planet, and an intelligence of some kind that can only survive by utilizing the few remaining sources of water supply. It is to the merit of Professor W. H. Pickering, to whom Professor Lowell gives the credit of having first suggested the idea of irrigation to account for the great width of the canals. What we see, then, is not the canal, which may be a slender stream of water, but a broad band of vegetation irrigated from these narrow channels. These lines penetrate and cross the dark regions in various directions, which again offer additional proof that the so-called seas are not seas but areas of vegetation sparsely scattered, against which the irrigated portions are of sufficient strength and color to show.[4]