HUMAN BIOLOGY 



Chapter I 



LIFE IN SPACE AND TIME 



Henry Norris Russell 



FEW problems arouse more widespread interest than 

 those of the extent of hfe in time and space. "Are 

 there other inhabited worlds than ours?" "How long 

 has hfe existed in this world?" "How long may it continue 

 to exist?" These are far from easy questions; but, by patient 

 accumulation of all the available data, a more definite 

 approach to an answer may now be given than seemed 

 possible a decade or two ago. 



We must first, of course, define our terms. "World" 

 may be used in its widest sense, to denote any body whose 

 existence can be observed, or rationally conjectured, within 

 the known universe. "Life," if we are to remain in the 

 realm of science, must be restricted to connote organisms 

 whose chemistry and metabohsm are of the same general 

 nature as those which are common to all terrestrial life, 

 animal and vegetable. Organisms of radically different 

 composition and structure may conceivably exist; but our 

 present knowledge of nature does not justify any extended 

 speculation concerning them. 



Thus specified, our problem falls naturally into successive 

 parts: First, we may enumerate those physical and chemical 

 conditions which appear to be necessary for the existence 

 and permanent maintenance of life of the kind that we 

 know. Second, we may review the bodies known to astron- 

 omy, considering which of them pass the tests just laid 

 down, and so may be the abodes of life, and what evidence 

 there is to believe that any of them actually are so. Third, 

 we may consider the evidence which bears on the past 

 duration of life where it now exists, and the probable interval 

 during which it may continue to flourish. Lastly, we have 

 to assess the probability that other habitable and inhabited 



3 



(ujjLIBRARy 



