O HUMAN BIOLOGY 



far below ioo°c. It is noteworthy, however, that some 

 organisms, such as the algae of the Yellowstone hot springs, 

 maintain themselves at temperatures but a few degrees 

 below the boiling point. How far life could follow the rise 

 of boiling point with higher pressure, we do not know. 

 It might even be that some resistant spores, or their equiv- 

 alent, might survive evaporation to dryness; but the 

 upper limit, as stated above, may serve as a basis for our 

 further discussion. 



(d) A Supply of Light Sufficient for Photosynthesis. Whether 

 this, again, is an absolute requirement is hard to determine, 

 for it is conceivable that organisms might derive their 

 supplies of energy from chemical reactions, starting with 

 inorganic materials; but the presence of light is certainly 

 highly favorable to the maintenance of Hfe, and probably 

 to its origin as well. Indeed, it has recently been shown 

 that light alone, in the presence of inorganic catalysts, 

 suffices for the photosynthesis of fairly complex compounds. 



Ultraviolet light needs special consideration. The shortest 

 waves transrnitted through the Earth's atmosphere (from 

 3500 to 3000 Angstrom units) are very potent physiologically, 

 and on the whole, highly beneficial. The shorter wave 

 lengths are very injurious, indeed lethal, which is doubtless 

 a consequence of the fact that terrestrial organisms are 

 never naturally exposed to them. That different limits of 

 tolerance could be developed under different conditions is 

 probable. 



The last two conditions demand that the abode of life 

 shall be a planet, revolving at an appropriate distance 

 about a self-luminous star, from which it derives its light 

 and heat. 



The conditions already stated appear to be either rigor- 

 ously necessary, or at the least, probably so, for the very 

 existence of life upon a planet. Those which follow, though 

 perhaps not unconditionally requisite for the development 

 and maintenance of hfe, are so much in its favor that they 

 should be added to the list. 



(e) The Existence of Land Areas. On a planet meeting the 

 other conditions of habitability, but bathed in a shoreless 

 ocean, life of suitable terrestrial forms might maintain 



