observed that many of the canals, without regard to duplication, were
invisible at times, and reappeared gradually; faint, scarcely visible
lines and shadows, deepened and became more distinct until they were
clearly and sharply defined, and these changes, likewise, were evidently
seasonal.

The invariable connection of the canals at their terminations with the
regions called seas, the fact that as the polar caps disappeared the
sealike expanses surrounding the polar regions deepened in color, and
other similar considerations soon led to the suggestion that there
existed on Mars a wonderful system of water circulation, whereby the
melting of the polar snows, as summer passed alternately from one
hemisphere to the other, served to reenforce the supply of water in the
seas, and, through the seas, in the canals traversing the broad
expanses of dry land that occupy the equatorial regions of the planet.
The thought naturally occurred that the canals might be of artificial
origin, and might indicate the existence of a gigantic system of
irrigation serving to maintain life upon the globe of Mars. The
geometrical perfection of the lines, their straightness, their absolute
parallelism when doubled, their remarkable tendency to radiate from
definite centers, lent strength to the hypothesis of an artificial
origin. But their enormous size, length, and number tended to stagger
belief in the ability of the inhabitants of any world to achieve a work
so stupendous.

After a time a change of view occurred concerning the nature of the
expanses called seas, and Mr. Lowell, following his observations of
1894, developed the theory of the water circulation and irrigation of
Mars in a new form. He and others observed that occasionally canals were
visible cutting straight across some of the greenish, or bluish-gray,