In the life history of at least one of the myriad star systems there has come a time when, on the surface of one of the minor members of the group, atoms of matter have been aggregated into such associations as to constitute what is called living matter. A question that at once suggests itself to any one who conceives even vaguely the relative uniformity of conditions in the different star groups is as to whether other worlds than ours have also their complement of living forms. The question has interested speculative science more perhaps in our generation than ever before, but it can hardly be said that much progress has been made towards a definite answer. At first blush the demonstration that all the worlds known to us are composed of the same matter, subject to the same general laws, and probably passing through kindred stages of evolution and decay, would seem to carry with it the reasonable presumption that to all primary planets, such as ours, a similar life-bearing stage must come. But a moment's reflection shows that scientific probabilities do not carry one safely so far as this. Living matter, as we know it, notwithstanding its capacity for variation, is conditioned within very narrow limits as to physical surroundings. Now it is easily to be conceived that these peculiar conditions have never been duplicated on any other of all the myriad worlds. If not, then those more complex aggregations of atoms which we must suppose to have been built up in some degree on all cooling globes must be of a character so different from what we term living matter that we should not recognize them as such. Some of them may be infinitely more complex, more diversified in their capacities, more widely responsive to the influences about them, than any living thing on earth, and yet not respond at all to the conditions which we apply as tests of the existence of life.