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 172 the lines are even more suggestive from a positive than they are from a negative standpoint. For they make continuations of the lines in the bright regions, showing that the two are causally connected, and affording strong presumption that this causal relation is the very one demanded by the theory of irrigation. For if the canals in the bright regions be strips of vegetation irrigated by a canal (too narrow to be itself visible at our distance), and there be a scarcity of water upon the surface of the planet, the necessary water would have to be conducted to the mouths of the canals across the more permanent areas of vegetation, thus causing bands of denser verdure athwart them, which we should see as dark lines upon the less dark background. Indeed, it is exactly what we should expect to find if the theory here advanced be true. For it is the very next logical step in that theory made visible. If the canals in the bright regions are to be fed from the melting of the polar cap, it is altogether likely that they would be connected with it by other canals running through the dark regions. We might, therefore, expect to see lines in the dark regions not unlike the lines in the bright ones, and if these lines were of the same character as those in the bright regions they would betray this character by connecting directly with them. Now this is precisely what he finds the two sets