our experience one other fact and one involving
much.

We know that our world is very old; that life has been for many
millions of years upon it; and that Man as a thinking being is
but of yesterday. Here is then a condition to be fulfilled. To
every world is physically assigned a limit to the period during
which it is habitable according to our knowledge of life and its
necessities. This limit passed and rationality missed, the chance
for that world is gone for ever, and other minds than ours
assuredly will not from it contemplate the universe. Looking at
our own world we see that the tree of life has,

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indeed, branched, leaved and, possibly, budded many times; it
never bloomed but once.

All difficulties dissolve and speculations become needless under
one condition only: that in which rationality may be inferred
directly or indirectly by our observations on some sister world
in space, This is just the evidence which in recent years has
been claimed as derived from a study of the surface of Mars. To
that planet our hope of such evidence is restricted. Our survey
in all other directions is barred by insurmountable difficulties.
Unless some meteoric record reached our Earth, revelationary of
intelligence on a perished world, our only hope of obtaining such
evidence rests on the observation of Mars' surface features. To
this subject we confine our attention in what follows.

The observations made during recent years upon the surface
features of Mars have, excusably enough, given rise to
sensational reports. We must consider under what circumstances
these observations have been made.

Mars comes into particularly favourable conditions for
observation every fifteen years. It is true that every two years
and two months we overtake him in his orbit and he is then in
"opposition." That is, the Earth is between him and the sun: he
is therefore in the opposite part of the heavens to the sun. Now
Mars' orbit is very excentric, sometimes he is 139 million miles
from the sun, and sometimes he as as much as 154 million miles
from the sun. The Earth's orbit is, by comparison, almost

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a circle. Evidently if we pass him when he is nearest to the sun
we see him at his best; not only because he is then nearest to
us, but because he is then also most brightly lit. In such
favourable oppositions we are within 35 million miles of him; if
Mars was in aphelion we would pass him at a distance of 61
million miles. Opposi