in form and function from any we know on this one small world of
ours. For just consider, Frida, what we mean by life. We mean a set
of simultaneous and consecutive changes going on in a complex mass
of organised carbon compounds. When most people say 'life,'
however,--especially here with you, where education is undeveloped--
they aren't thinking of life in general at all (which is mainly
vegetable), but only of animal and often indeed of human life.
Well, then, consider, even on this planet itself, how special are
the conditions that make life possible. There must be water in some
form, for there's no life in the desert. There must be heat up to
a certain point, and not above or below it, for fire kills, and
there's no life at the poles (as among Alpine glaciers), or what
little there is depends upon the intervention of other life wafted
from elsewhere--from the lands or seas, in fact, where it can
really originate. In order to have life at all, as WE know it at
least (and I can't say whether anything else could be fairly called
life by any true analogy, until I've seen and examined it), you
must have carbon, and oxygen, and hydrogen, and nitrogen, and many
other things, under certain fixed conditions; you must have liquid
water, not steam or ice: you must have a certain restricted range
of temperature, neither very much higher nor very much lower than
the average of the tropics. Now, look, even with all these
conditions fulfilled, how diverse is life on this earth itself, the
one place we really know--varying as much as from the oak to the
cuttle-fish, from the palm to the tiger, from man to the fern, the
sea-weed, or the jelly-speck. Every one of these creatures is a
complex result of very complex conditions, among which you must
never forget to reckon the previous existence and interaction of
all the antecedent ones. Is it probable, then, even a priori, that
if life or anything like it exists on any other planet, it would