If we consider man alone, we find him at Aden, on the Red Sea, at a temperature of 130° in the shade, and in Siberia at 70° below zero; grovelling in mines deep in the Earth, and living in great communities ten thousand feet above sea-level; fighting battles on the slopes of the Himalayas, at an altitude of 19,000 feet; nomadic on sterile tracts; sweltering under the glaring sun of the equator, and existing in regions of perpetual snow and ice, and without sunlight for six months of the year. Such are a few of the varied conditions to which man has become accustomed since he emerged from his tropical and arboreal relatives.
The question finally comes down to the effect of the rarefaction of air on life. An inquiry as to how far man can stand changes of atmospheric pressure is of interest in this connection, for we know that sudden changes are accompanied by mountain sickness, at great elevations, and caisson disease under great pressure. Large birds soar among the high peaks of the Andes and drop at once to sea-level. I have dredged delicate mollusks at a depth of one hundred and fifty fathoms of water and kept them alive for weeks in an aquarium. Man, while showing a sensitiveness to changes in barometric pressure when experienced suddenly, can nevertheless get accustomed to great ranges of pressure. The cities of Bogota and Quito are 10,000 feet above the level of the sea and yet in Quito when De Saussure, the naturalist, became so ill from the rarefaction that he could hardly find energy enough to read his instruments, and his servants, digging holes in the snow, fainted from the exertion, the natives were pursuing their various activities, and bull-fights were going on! One has only to read the accounts of the English expedition to Thibet to learn that troops fought in skirmishes at the height of 19,000 feet.
Mr. Douglas W. Freshfield (in "Scot. Geo. Mag.," April, 1905) gives an account of mountain sickness in the Sikkim Himalaya. He says the effect of high altitude was different in different individuals; some men were entirely free from it, and among them a Goorkha, who ran back in a pass at an altitude of 20,000 feet to hurry up some loiterers. Another member of the party, an Englishman, actually gained in weight, and had an increased appetite. Here, then, are a few men among a small number, without previous experience in rarefied air, feeling no disturbance, and, in one case, actually benefited by it!
The question arises as to what natural selection would do among a hundred million say, who, through many centuries, might be subject to a gradual attenuation of the air. The result of rarefaction of the atmosphere and the absence of moisture is associated with marked hygienic influences. The Hadley Climatological Laboratory of the University of New Mexico has made special investigations as to the increased lung capacity of those living at high altitudes, the relation of dry soil to health, etc. Important work has been done by Drs. John Weinzirl, C. Edw. Magnusson, F. S. Maltby, and Mrs. W. C. Hadley, and their investigations go to prove that high altitudes and absence of moisture are favorable to the health of man on this world, and by analogy would not be inimical to the survival of certain forms of life in Mars.
Dr. S. E. Solby (in "Medical Climatology," p. 43, 1897), in describing the effects of rarefaction of the air says: "The amount of air taken in at each breath becomes greater, and the air-cells, many of which are at lower altitudes often unused, are dilated."
If we consider the atmospheric pressure under which a man can work and live, we find equal adaptability.
Mr. Gardner D. Hiscox, in his work on "Compressed Air, Its Production, Uses, and Applications," says: "Experience has taught that the ill effects are in proportion to the rapidity with which the transmission is made from compressed air to the normal atmosphere. That while the pressure remains stationary all subjective phenomena disappear." He speaks of pressure of forty or fifty pounds to the square inch, and says that, at these pressures, taste, smell, and the sense of touch lose their acuteness.
In the "Engineering Record" for January 23, 1904, there is an interesting article on "Caisson Disease." It says that twenty pounds pressure per square inch is common on foundation work in New York, and that bridge piers have been built when pressures of nearly fifty pounds were required. The deepest pneumatic work in New York was done in the East River gas tunnel, when the maximum pressure was about forty-seven to fifty pounds per square inch above atmospheric. In the gas tunnel four men died from the effects of heavy pressure, while none died from that reason under bridge work. The article further says that ordinarily "strong young men in proper condition do not suffer from working two four-hour shifts daily, under pressure up to twenty-five or thirty pounds; above that limit injurious effects may be felt," etc.
Let any reasonable man consider the meaning of these data. Without any selective action on the race, without even a graded increase of pressure from boyhood up, these workmen perform hard labor of stone excavation at these pressures, and in the same way, without previous experience, men are fighting battles at 19,000 feet altitude, and in one instance growing fat at 20,000 feet. Eminent German and French scientists have studied the effects of pneumatic pressure by numerous experiments on men and animals. One experimenter subjected a great number of dogs, cats, rabbits, guinea-pigs, and other animals to repeated pressures up to one hundred pounds, and carefully observed the effects of the varying conditions, some of which were fatal, while others were apparently harmless. The experiments showed that sudden release from heavy pressures was fatal, but that if three or four hours were occupied in reducing a pressure of one hundred pounds, it was harmless.
With these facts one cannot help wondering whether even man himself could not exist on Mars if allowed time to get accustomed to the rare atmosphere through thousands of generations of minute increments of adaptation.