Popular interest in astronomy is exceedingly wide, but it is very largely confined to the idea of resemblances and differences between our earth and the bodies of the sky. The question most frequently asked the astronomer is, "Have any of the stars got people on them?" Or more specifically, "Is Mars inhabited?" The average questioner will not readily be turned off with yes or no for an answer. He may or may not know that it is quite impossible for astronomers to ascertain anything definite in this matter, most interesting as it is. What he wants to find out is the view of the individual astronomer on this absorbing and ever recurring inquiry. We ought first to understand what is meant by the manifestation here on the earth called life, and agree concerning the conditions that render it possible. Apparently they are very simple. We may or may not agree that a counterpart of life, or life of a wholly different type from ours, may exist on other planets under conditions wholly diverse from those recognized as essential to its existence here. The problem of the origin of life is, in the present state of knowledge, highly speculative and hardly within the domain of science. Here on earth, life is intimately associated with certain chemical compounds, in which carbon is the common element without which life would not exist. Also hydrogen, oxygen, and nitrogen are present, with iron, sulphur, phosphorus, magnesium and a few less important elements besides. But carbon is the only substance absolutely essential. Protoplasm cannot be built without it, and protoplasm makes up the most of the living cell. Closely related to carbon is silica also, as a substitution in certain organic compounds. Protoplasm is able to stand very low temperatures, but its properties as a living cell cease when the temperature reaches 150 Fahrenheit. Animal life as it exists on the earth to-day appears to have been here many million years. The palæontologists agree that all life originated in the waters of the earth. It has passed through evolutionary stages from the lowest to the highest. Throughout this vast period the astronomer is able to say that the conditions of the earth which appear to be essential to the maintenance of life have been pretty constantly what they are to-day. The higher the type of life, the narrower the range of conditions under which it thrives. Man can exist at the frigid poles even if the temperature is 75 degrees below Fahrenheit zero; and in the deserts and the tropics, he swelters under temperatures of 115 degrees, but he still lives. At these extremes, however, he can scarcely be said to thrive.