From preference, however, these lowest forms of life affect what to us would be unbearable temperatures. Many of the chromacea live in hot springs at temperatures of 123° to 176° Fahrenheit, in which no other, that is, no higher, organism can dwell. This choice of habitat is in line with the other details of their evolutionary career. For it, too, is in keeping with the conditions of crystalline growth, halfway as it were on the road to them; the forming of crystals beginning at a temperature higher still. And we perceive from it that the passing of the inorganic into the organic is brought about by a lowering of the temperature of the parent planet. This again, is in line with the evolution of chemical complexity. Let the heat become less, and higher and higher chemical compounds, finally the organic ones, become possible. That evolution is nothing else than such a gradually increasing chemical synthesis is forced on one by study of the facts. Once started, life, as paleontology shows, develops along both the floral and the faunal lines side by side, taking on complexity with time. It begins so soon as secular cooling has condensed water vapor to its liquid state; chromacea and confervæ coming into being high up toward the boiling-point. Then, with lowering temperature come the seaweeds and the rhizopods, then the land plants and the lunged vertebrates. Hand in hand the fauna and flora climb to more intricate perfecting, life rising as temperature lowers. We perceive then that, considered _a priori_, the possibility of life on a planet is merely a question of the planet’s size; and then pursuantly that the character of that life is a matter of the planet’s age. But age again is a question of size. For the smaller its mass the quicker the body cools, and with a planet, growing cold means growing old. Within the bounds that make life possible, the smaller the body the quicker it ages and the more advanced its denizens must be. Just how far the advance goes we may not assert dogmatically in a given case, since not relative age alone but absolute time as well is concerned in it. It may be that nature’s processes cannot be hurried, and that for want of time development may in part be missed. But from general considerations the limit of the time needed seems well within most planetary careers.