be conceived to have grappled with the stupendous problem of keeping
their world in a habitable condition as long as possible. Supposing
them to have become accustomed to live in their rarefied atmosphere (a
thing not inconceivable, since men can live for a time at least in air
hardly less rare), the most pressing problem for them is that of a
water-supply, without which plant life cannot exist, while animal life
in turn depends for its existence upon vegetation. The only direction
in which they can seek water is that of the polar regions, where it is
alternately condensed into snow and released in the liquid form by the
effect of the seasonal changes. It is, then, to the annual melting of
the polar snow-fields that the Martian engineers are supposed to have
recourse in supplying the needs of their planet, and thus providing
the means of prolonging their own existence. It is imagined that they
have for this purpose constructed a stupendous system of irrigation
extending over the temperate and equatorial regions of the planet. The
``canals'' represent the lines of irrigation, but the narrow streaks
that we see are not the canals themselves, but the irrigated bands
covered by them. Their dark hue, and their gradual appearance after
the polar melting has begun, are due to the growth of vegetation
stimulated by the water. The rounded areas visible where several
``canals'' meet and cross are called by Mr Lowell ``oases.'' These are
supposed to be the principal centers of population and industry. It
must be confessed that some of them, with their complicated systems of
radiating lines, appear to answer very well to such a theory. No
attempt to explain them by analogy with natural phenomena on the earth
has proved successful.

But a great difficulty yet remains: How to explain the seemingly
miraculous powers of the supposed engineers? Here recourse is had once
more to the relative smallness of the planet. We have remarked that