LIFE IN SPACE AND TIME 



itself indefinitely if transplanted under favorable conditions. 

 But how life could have originated on such a planet is a 

 hard question. Without trespassing on the field of the 

 biologist, reference may be made to the belief that the 

 appearance of such complex systems as the simplest of 

 hving things is far more likely to occur if there exists a 

 large number of more or less isolated local environments, 

 such as would be provided by tide-pools or fresh-water 

 ponds, than in the uniform conditions of the open sea. 

 Indeed, in a sea so deep that no hght reached the bottom, 

 it is very hard to see how life could get any start. The pres- 

 ence of land, that is the absence of excess of water, appears 

 therefore to be almost a necessary condition. 



(f) Rotation of the Planet. This again is hardly a sine 

 qua non, but, nevertheless, important. The regular alterna- 

 tion of day and night, and the seasonal changes which 

 accompany it if the equator is inchned to the plane of the 

 orbit, greatly increase the area of the planet's surface over 

 which favorable temperature conditions are attained, and 

 the rhythmic alternation of the environment is pretty 

 well recognized as a favorable factor in evolution. The 

 effective alternative to rotation is of course a state in which 

 the planet keeps the same face always toward its primary — 

 as the Moon does toward the Earth, or Mercury toward 

 the Sun. Under these conditions, the range of temperature 

 from one side of the planet to the other will be very great, 

 and the atmospheric circulation probably very violent, 

 and the conditions, though not necessarily fatal to Hfe, 

 will be clearly unfavorable. 



(g) Atmospheric Oxygen. Free oxygen is a prime necessity 

 for animal life, and a waste product of most vegetable forms. 

 One cannot exist without consuming, nor the other without 

 producing it. Yet, though it is so intimately associated with 

 the higher forms of life, this is not the case with the lower. 

 Many bacteria, for instance, can grow only in its absence. 

 There appears, therefore, to be no sufficient reason for 

 laying down the presence of free oxygen as a prerequisite 

 for the origin of life upon a planet. Other chemical reactions 

 than those involving direct oxidation might have provided 

 the primordial forms of life with the required energy. 



