Biologists tell me that they hardly know enough regarding the extreme adaptability of organisms to environment to enable them to say whether life on such a planet as Mars would or would not keep on functioning with secular changes of moisture and temperature. The survival of a race might be insured against extremely low temperatures by dwelling in sub-Martian caves, and sufficient water might be preserved by conceivable engineering and mechanical schemes; but the secular reduction of the quantity and pressure of atmosphere--it is not easy to see how a race even more advanced than ourselves could maintain itself alive under serious lack of an element so vital to existence. Both Wallace, the great biologist, and Arrhenius, the eminent chemist (but biologist, astronomer, and physicist as well), both reject the habitation theory of Mars, regarding the so-called canals as quite like the luminous streaks on the moon; that is, cracks in the volcanic crust caused by internal strains due to the heated interior. Wallace, indeed, argues that the planet is absolutely uninhabitable. The asteroids, or minor planets? We may dismiss them with the simple consideration that their individual masses are so insignificant and their gravity so slight that no atmosphere can possibly surround them. Their temperatures must be exceedingly low, and water, if present at all, can only exist in the form of ice. Jupiter, the giant planet, presents the opposite extreme. His mass is nearly a thousandth part of the sun's, and is sufficient to retain a very high temperature, probably approximating to the condition we call red-hot. This precludes the possibility of life at the outset, although the indications of a very dense atmosphere many thousand miles in depth are unmistakable. Of Saturn, one thirty-five hundredth the mass of the sun, practically the same may be said. Proctor thought it quite likely that Saturn might be habitable for living creatures of some sort, but he regarded the planet as on many accounts unsuitable as a habitation for beings constituted like ourselves. Mere consideration of surface temperature precludes the possibility of life in the present stage of Saturn's development; but the consensus of opinion is to the effect that life may make its appearance on these great planets at some inconceivably remote epoch in the future when the surface temperature is sufficiently reduced for life processes to begin. Discoveries of algæ flourishing in hot springs approaching 200 degrees Fahrenheit make it possible that these beginnings may take place earlier and at much higher temperatures than have hitherto been thought possible.