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8 one, the sum total from the four becomes not seventeen to one but three hundred to one in favor of its being true. It will be seen how rapidly the probability of the truth of a theory mounts up from the amount of detail it explains. This law is to be remembered throughout the coming exposition, for whatever the cogency of each detail of the argument in itself, the concurrence of all renders them not simply additionally but multiplicitly effective. That different lines of induction all converge to one point proves that point to be the radiant point of the result.

To determine whether a planet be the abode of life in at least resembling that with which we are acquainted, two questions about it must be answered in turn: first, are its physical conditions such as render it, in our general sense, habitable; and secondly, are there any signs of its actual habitation? These problems must be attacked in their order, for unless we can answer the first satisfactorily, it were largely futile to seek for evidence of the second.

Thoroughly to appreciate, then, the physical condition of Mars, we must begin at the beginning of our knowledge of the planet, since every detail will be found to play its part in the final result. I shall therefore give in a word or two