In constructing a similar diagram for Mars, three modifications have to be made. First of all, the mean temperature of the planet must be considerably lower than that of the Earth. Next, since the atmospheric circulation is languid and there are no great oceans, the temperatures of different latitudes cannot be equalized to the same extent as on the Earth. It follows, therefore, that the range in mean temperature from equator to pole must be considerably greater on Mars than on the Earth. Thirdly, the range in temperature in any latitude, from the hottest part of the day in summer to the coldest part of the night in winter, must be much greater than with us; partly on account of the very slight density of the atmosphere, and partly on account of the length of the Martian year. [Illustration: THERMOGRAPHS OF THE EARTH AND MARS] We cannot know the exact figures to adopt, but the general type of the thermograph for Mars as compared with that of the Earth will remain. The mean temperature of Mars will be lower, the range of temperature from equator to pole will be greater, and the extremes of temperature in any given latitude more pronounced than upon the Earth. And the general lesson of the diagram may be summed up in a sentence. The maximum temperature on the planet is well above freezing-point, and the part of the planet at maximum temperature is precisely the part that we see the best. But while this is so, it is clear that water on Mars must normally be in the state of ice; Mars is essentially a frozen planet; and the extremes of cold experienced there, not only every year but every night, far transcend the bitterest extremes of our own polar regions. The above considerations do not appear to render it likely that there is any vegetation on Mars. A planet ice-bound every night and with its mean temperature considerably below freezing-point does not seem promising for vegetation. If vegetation exists, it must be of a kind that can pass through all the stages of its life-history during the few bright hours of the Martian day. Every night will be for it a winter, a winter of undescribable frost, which it could only endure in the form of spores. So if there be vegetation it must be confined to some hardy forms of a low type. At a distance of forty millions of miles it is not easy to discriminate between the darkness of sheets of water and the darkness of stretches of vegetation. Some of the so-called "seas" may possibly be really of the latter class, but that there must be expanses of water on the planet is clear, for if there were no water surfaces there would be no evaporation; and if there were no evaporation from whence could come the supply of moisture that builds up the winter pole cap?