That all or any of the conditions considered would be complied with in the case of a planet revolving round a star it is, of course, impossible to say. But when we find stars showing by their spectra that they contain chemical elements identical with those which exist in the sun and the earth, analogy would lead us to suppose that very possibly a planet resembling our earth may revolve round each of these distant suns. I say _a_ planet, for evidently there would be only _one_ distance from the central luminary—a distance depending on its size—at which the temperature necessary for the support of life would exist, as in the case of the earth, over the whole of the planet’s surface. For other planets of the stellar system, life would be, if it existed at all, most probably confined to restricted regions of the planet’s surface. There would, therefore, be in each system one planet, and only one, _especially_ suitable for the support of animal life as we know it. This is with reference to light and heat. If the other conditions were not complied with, then life would probably not exist even on this one planet. In the case of a star larger than the sun, the planet should be placed at a greater distance than the earth is from the sun, but in this case the length of the year and the seasons would be longer than ours. The star which more nearly resembles the sun in the character of the light which it emits is the bright star Capella. Arcturus has a somewhat similar spectrum. But these are probably suns of enormous size, if any reliance can be placed on the measures of their distance from the earth. Other bright stars with spectra of the solar type are Pollux, Aldebaran, Beta Andromedæ, Alpha Arietis, Alpha Cassiopeiæ, Alpha Cygni, and Alpha Ursæ Majoris. Another star is Eta Herculis. The magnitude of this star as measured with the photometer is about 3½. A parallax found by Bélopolsky and Wagner places it at a distance of 515,660 times the sun’s distance from the earth. If the sun were placed at this distance, I find that it would be reduced to a star of the third magnitude. This result would imply that Eta Herculis is a slightly smaller sun than ours; and a planet placed a little nearer to the star than the earth is to the sun might, perhaps, fulfil the conditions of a life-bearing world. The number of stars visible in our largest telescopes is usually estimated at 100,000,000. Of these we may perhaps assume that 10,000,000 have a spectrum of the solar type, and therefore closely resemble our sun in their chemical constitution. If we suppose that only one in ten of these is similar in size to the sun, and has a habitable planet revolving round it, we have a total of 1,000,000 worlds in the visible universe fitted for the support of animal life. We may therefore conclude, with a high degree of probability, that among the “multitudinous” stellar hosts there are probably many stars having life-bearing planets revolving round them.