areas that had been regarded as seas. This fact suggested that, instead
of seas, these dark expanses may rather be areas of marshy ground
covered with vegetation which flourishes and dies away according as the
supply of water alternately increases and diminishes, while the reddish
areas known as continents are barren deserts, intersected by canals; and
as the water released by the melting of the polar snows begins to fill
the canals, vegetation springs up along their sides and becomes visible
in the form of long narrow bands.

According to this theory, the phenomena called canals are simply lines
of vegetation, the real canals being individually too small to be
detected. It may be supposed that from a central supply canal irrigation
ditches are extended for a distance of twenty or thirty miles on each
side, thus producing a strip of fertile soil from forty to sixty miles
wide, and hundreds, or in some cases two or three thousands, of miles in
length.

The water supply being limited, the inhabitants can not undertake to
irrigate the entire surface of the thirsty land, and convenience of
circulation induces them to extend the irrigated areas in the form of
long lines. The surface of Mars, according to Lowell's observation, is
remarkably flat and level, so that no serious obstacle exists to the
extension of the canal system in straight bands as undeviating as arcs
of great circles.

Wherever two or more canals meet, or cross, a rounded dark spot from a
hundred miles, or less, to three hundred miles in diameter, is seen. An
astonishing number of these appear on Mr. Lowell's charts. Occasionally,
as occurs at the singular spot named Lacus Solis, several canals
converging from all points of the compass meet at a central point like