When imaginative men turn their eyes towards space and wonder whether life exists in any p

art of it, they may cheer themselves by remembering that life need not resemble closely the life that exists on Earth. Mars looks like the only planet where life like ours could exist, and even this is doubtful. But there may be other kinds of life based on other kinds of chemistry, and they may multiply on Venus or Jupiter. At least we cannot prove at present that they do not.

Even more interesting is the possibility that life on their planets may be in a more advanced stage of evolution. Present-day man is in a peculiar and probably temporary stage. His individual units retain a strong sense of personality. They are, in fact, still capable under favorable circumstances of leading individual lives. But man's societies are already sufficiently developed to have enormously more power and effectiveness

than the individuals have.

It is not likely that this transitional situation will continue very long on the evolutionary time scale. Fifty thousand year's from now man's societies may have become so close-knit that the individuals retain no sense of separate personality. Then little distinction will remain between the organic parts of the multiple organism and the inorganic parts (machines) that have been constructed by it. A million years further on man and his machines may have merged as closely as the muscles of the human body and nerve cells that set them in motion.

The explorers of space should be prepared for some' such situation. If they arrive on a foreign planet that has reached an advanced stage (and this is by no means impossible), they may find it being inhabited by a single large organism composed of many closely cooperating units.

The units may be "secondary"-machines created millions of years ago by a previous form. of life and given the will and ability to survive and reproduce. They may be built entirely of metals and other durable materials. If this is the case, they may be much more tolerant of their environment, multiplying under conditions that would destroy immediately any organism made of carbon compounds and dependent on the familiar car bon cycle.

Such creatures might be relics of a past age, many millions of years ago, when their planet was favorable to the origin of life, or they might be immigrants from a favored planet.

Humans on Earth today are characterized by______.

A．their existence as free and separate beings

B．their capability of living under favorable conditions

C．their great power and effectiveness

D．their strong desire for living in a close-knit society