THE SUPPLY OF OXYGEN 103 



atmosphere. There is at least twenty times as much 

 oxygen in a bottle of air as in the same quantity 

 of fresh or salt water. When we consider this, then 

 the fact that all the higher forms of life, plant and 

 animal, are aerial and the lower mainly aquatic becomes 

 more intelligible. The more complex frame of the 

 advanced classes of life demands more oxygen than 

 that of the simple, both on account of the speedier 

 rate at which their tissues disintegrate and re-form, 

 and also on account of the more rapid and laborious 

 work that their movements imply. The greater size 

 as well as complexity to which land plants and animals 

 attain is another reason for their more active respira- 

 tion. The spacious air and abounding waters provide 

 an unlimited store of the requisite oxygen. Unlike 

 the search for food, which is a quest beset with 

 difficulty and uncertainty, the breath of life is around 

 all that live upon the earth and bounteous beyond 

 computation. At the surface of the earth, or that of 

 the hills, its richness is greatest, and as we descend to 

 the waters its bounty diminishes. Above and below 

 the surface the amount of oxygen varies. If we climb 

 beyond the snow-line this change causes mountain- 

 sickness ; if we enter a cave our lights burn low. As 

 with the surface of the earth so with the surface waters. 

 The upper strata are rich in oxygen from wave-move- 

 ment and the imprisoned air of the foam ; and this 

 bounty is reflected in the teeming life and comparatively 

 high development of its fauna ; below in the stillness 

 there is poverty of oxygen, only matched by the cave 



