and geology exhibits to us adaptations much more wonderful, so that even a considerable deviation from the normal conditions would not invalidate the analogical inference. It ought to be kept in view, however, that there must be a limit to the variation, otherwise the argument ceases to be one of analogy. The argument is founded upon similarity, but if the variation is pushed to such an extent as to obliterate the resemblance, the logical inference is vitiated. But how are we to limit the variation, and yet to preserve the analogical character of the argument? We must assign limits from our knowledge of the variations with which we are acquainted. We see a wide range on our globe, but we can also detect a limit. We find, for example, that life is consistent with a wide range of density of atmosphere, but we have no reason to believe that life could exist on our globe without an atmosphere. There is evidently a limit far short of total deprivation. So in regard to temperature, there are vast wastes on the face of the earth where life cannot exist on account of the extremes of temperature. But it may be said, why should the limits of our globe be applied to the other planetary bodies? May not this variation be so great that life might exist in a comet some thousand times hotter than white-hot iron; or on a globe without atmosphere, and with a temperature below zero? But to argue in this way is to give up entirely the analogical argument, which requires us to
