THE ENVIRONMENT 51 



nourishing than the rare molecules of a nebula, 

 or the still rarer particles of interstellar space, 

 must be provided. 



We may safely conclude, therefore, on the 

 basis of our reliable knowledge of the universe, 

 that active life can exist probably only upon 

 a dense, crusted body 1 ; for, of course, the in- 

 terior of the earth is no better suited to life 

 than is the interior of the sun. 



We have perhaps taken a long road to arrive 

 at so familiar an idea. But our task involves 

 the consideration of every conceivable form 

 of life, not merely that relatively anthropo- 

 morphic kind which we commonly think 

 of when speculating loosely regarding life in 

 other worlds. 



It is indeed possible that the common-sense 

 judgment of the universe which declares our 

 solar system to be on the whole, in its funda- 

 mental characteristics, typical of every such 

 system may turn out to be in some respects 

 unjustified. For the present, however, so 

 long as we use it only as an indication of the 

 direction in which we are to turn our atten- 

 tion, there is certainly no risk whatever in fol- 

 lowing this hypothesis in the later discussion. 



1 For an interesting discussion of the necessary conditions 

 of existence see P. Lowell, "Mars as the Abode of Life." 

 The Macmillan Company, New York, 1908. 



