24 HUMAN BIOLOGY 



animals, may exist on Mars; but we have no reliable evidence 

 whether or not they do. 



It is interesting to consider that, if Venus had intelHgent 

 inhabitants, who could observe the Earth as well as we 

 can study Mars, and whose instrumental equipment and 

 scientific knowledge were at the present human level, 

 these inhabitants would probably come to just the same 

 conclusion regarding the Earth. Seasonal changes on our 

 planet would be conspicuous; the presence of oxygen would 

 be spectroscopically evident; the widespread existence of 

 vegetation might be inferred; but of the works of man, even 

 his greatest, nothing could be seen which could definitely 

 be distinguished from the products of unconscious forces. 

 The reahzation of this is wholesome, for us the self-styled 

 "lords of creation." 



THE AGE OF THE EARTH AND OF THE STARS 



Geologists have long been able to arrange the strata 

 in order of their relative age, and to distinguish many 

 successive periods by their characteristic fossils; but it is 

 only in recent times that the geological time-scale has 

 become expressible in years. 



Estimates based upon such processes as the date of 

 formation of sediments, or that of accumuLation of soluble 

 salts in the ocean suffer from the difficulty that, even 

 though the present rate at which these proceed, and the 

 cumulative effect of their action in past time, were accurately 

 known, we have no security that their rate of operation 

 in the past was of the same magnitude as at present. Calcula- 

 tions based on the coohng of the Earth's interior once 

 appeared more reliable, but they have been completely 

 upset by the discovery that heat is generated by radioactive 

 substances in the superficial layers. Radioactivity, however, 

 has furnished us with what is apparently a trustworthy 

 time-scale in place of these dubious ones. 



The radioactive elements are gradually and spontaneously 

 disintegrating, an atom of one sort ejecting a part of itself 

 and changing into an atom of another, and so on through 

 a long sequence of transformations. The rate of these trans- 

 formations appears to be quite unaffected by any external 



