The theory that intelligent men exist on Mars is very popular. With its help everything may be explained, particularly if we attribute an intelligence vastly superior to our own to these beings, so that we not always are able to fathom the wisdom with which their canals are constructed. The crossings of the latter are said to be cities (Lowell) fifty times greater than London. The trouble with these “explanations” is that they explain anything, and therefore in fact nothing. If we would endeavour to understand the phenomena on Mars, we must in the first place avoid the formerly so popular principle of “purposiveness” which led even the most prominent scientists into so many amusing errors. Neither may we base our conceptions, as does Flammarion, on the assumption of natural forces unknown to us, no matter how much such a course may appeal to mystics. Only forces with which we are familiar can be resorted to, if we really are to understand nature. It seems to me that such method of research might with good results be applied also to the planet Mars. CHAPTER VII MERCURY, THE MOON, AND VENUS The planet Mercury probably resembles Mars in many respects, but differs particularly in lacking an atmosphere. The fissures in the crust of the Earth or of Mars are as a rule rapidly filled and their contours largely hidden from sight by alluvium or sand carried by windstorms, so that they reveal themselves only through tremors and various emanations along their course. The fissures on Mercury on the other hand must remain as yawning chasms. It is probable that reducing gases stream out of these cracks as on Earth, and colour the environment in a darker shade than the other visible part of the planet’s surface, that is the hemisphere turned toward the Sun. Not very volatile gases, such as sal-ammoniac, other chlorides and sulphur, which on the Earth are deposited inside the fissures, may here spread over large areas and discolour surrounding territory, particularly where iron compounds are present, and, under the attack of sulphur, turn black. Lowell has made drawings of the dark spots visible on Mercury, one of which is reproduced as Fig. 25. These spots lie, as on Antoniadi’s drawing of the surface of Mars (Fig. 17a), arranged in lines which are almost straight or of a slight curvature only. This seems to indicate that the spots belong to areas immediately adjoining enormous fissures. According to Lowell’s drawing, these cracks are far more regularly distributed on Mercury than on Mars. Very close to the centre of the ever-sunny side we see a dark spot, a “lake.” It is evident that this spot is located in by far the hottest point on the surface of Mercury. This gives rise to the following conception. The hottest part of Mercury was naturally the last to solidify. Mercury evidently ceased to rotate around its own axis, leaving one side continually exposed to the Sun, while its surface yet consisted of