20 HUMAN BIOLOGY 



to see the sharp geometrical network; and many intermediate 

 drawings and descriptions exist. 



There is no room here for a full discussion of this problem; 

 but it may be said in brief that the only reasonable explana- 

 tion of these discrepancies appears to he in what astronomers 

 call "personal equation." The process of recording, by 

 sketch or verbal description, details which can be seen 

 only by ghmpses of a few seconds' duration when the air 

 is steady, is one of extreme complexity; and psychological 

 elements in the report which the eye makes to consciousness 

 are apparently important. The most skilled and scrupulous 

 observers cannot discriminate between the objective and 

 subjective elements in such a report of his senses, and so the 

 discrepancies arise. 



There is little reason to hope that keener eyes, or better 

 atmospheric conditions, will be available for work on Mars 

 in the future than there have been in the past, and it appears 

 necessary to leave the question of the exact appearance and 

 arrangement of the finer Martian markings sub judice, 

 as a matter not at present determinable; while admitting 

 their reality. The photographs, unfortunately, cannot 

 settle the question, for the details in question are so fine 

 that the grain of the plates would conceal them, even if no 

 other difficulties existed. 



All the necessary and important conditions favorable 

 to life appear to be present on the surface of Mars: an 

 adequate temperature, sunlight, water, atmospheric oxygen, 

 a land surface, days and seasons. The force of gravity 

 at the surface, and the atmospheric density, though less 

 than those to which terrestrial life is adapted, appear 

 to be well within the limits of possible adaptation. 



Two independent lines of evidence point toward the 

 actual existence of vegetable life upon the planet. The 

 first is the character of the seasonal changes in the dark 

 areas and canals. The brief description already given shows 

 that these are just what might be expected from the growth 

 of vegetation, as the temperature rises in spring, and the 

 water locked up in the polar cap is released, and its dying 

 down in the cold and dryness of late autumn and winter. 

 The fact that the changes, in successive Martian years, 



