The best clue to a natural interpretation of the strange features of the surface of Mars is that suggested by the American astronomer Mr. W.H. Pickering in _Popular Astronomy_ (1904). Briefly it is, that both the 'canals' of Mars and the rifts as well as the luminous streaks on the moon are cracks in the volcanic crust, caused by internal stresses due to the action of the heated interior. These cracks he considers to be symmetrically arranged with regard to small 'craterlets' (Mr. Lowell's 'oases') because they have originated from them, just as the white streaks on the moon radiate from the larger craters as centres. He further supposes that water and carbon-dioxide issue from the interior into these fissures, and, in conjunction with sunlight, promote the growth of vegetation. Owing to the very rare atmosphere, the vapours, he thinks, would not ascend but would roll down the outsides of the craterlets and along the borders of the canals, thus irrigating the immediate vicinity and serving to promote the growth of some form of vegetation which renders the canals and oases visible.[13] [Footnote 13: _Nature_, vol. 70, p. 536.] This opinion is especially important because, next to Mr. Lowell, Mr. Pickering is perhaps the astronomer who has given most attention to Mars during the last fifteen years. He was for some time at Flagstaff with Mr. Lowell, and it was he who discovered the oases or craterlets, and who originated the idea that we did not see the 'canals' themselves but only the vegetable growth on their borders. He also observed Mars in the Southern Hemisphere at Arequipa; and he has since made an elaborate study of the moon by means of a specially constructed telescope of 135 feet focal length, which produced a direct image on photographic plates nearly 16 inches in diameter.[14] [Footnote 14: _Nature_, vol. 70, May 5, p.xi, supplement.] It is clear therefore that Mr. Lowell's views as to the artificial nature of the 'canals' of Mars are not accepted by an astronomer of equal knowledge and still wider experience. Yet Professor Pickering's alternative view is more a suggestion than an explanation, because there is no attempt to account for the enormous length and perfect straightness of the lines on Mars, so different from anything that is found either on our earth or on the moon. There must evidently be some great peculiarity of structure or of conditions on Mars to account for these features, and I shall now attempt to point out what this peculiarity is and how it may have arisen. _The Meteoritic Hypothesis._