OUR PLACE IN SPACE 363 



same. If it had not been so in the beginning, we shall see 

 that there is reason to believe it would become so in the 

 course of any extended period of existence. If the stuff is 

 the same, doubtless the properties inherent to matter are the 

 same. We may scarcely regard our solar system as unique. 

 We are led almost irresistibly to conclude that we may have 

 many similars throughout the concourse of worlds. 



Let us conceive for a moment that there are in the sky no 

 more, or that we shall never know of any more, than say a billion 

 suns a thousand millions. If our solar system be anything of 

 an average type, this would mean five or ten billion planets 

 that might be, at some period or other of their cooling-off, the 

 seat of an animal population, perhaps of a civilisation and an 

 intellectual development something akin to our own. The 

 number is large. It can hardly excite in the mind more than 

 a vacant mental stare. But consider the further inference. 

 We have as yet, of course, only the vaguest estimates of the 

 age of our planetary system, in the sense that it comprised 

 definitely defined bodies. Present-day estimates range from 

 twenty-five or fifty millions of years, up to several hundred. 

 Let us take a figure far outside, and call it a thousand millions. 



Again, we may conceive that at some point or other over 

 the earth our human civilisation has held at about its present 

 level for ten or twenty thousand years. So far as primitive 

 intellectual power is concerned, or in social organisation, in his 

 pomps and ceremonies, his dress and his ways, man has pro- 

 bably varied but little within this period. The time may be 

 much longer. It is now clear that it was not less. 



Such minds as love to follow out calculations of probability 

 will not fail to run forward to the conclusion. Let us say 

 that the civilised life of mankind represents a hundred- 

 thousandth part of the conceivably habitable period of our 

 solar system, in any portion of it, and assume this system to 

 be a universal type. It follows that it would need but a universe 

 of a hundred thousand suns to conceive that there is at least 

 one other existing planet which has reached a stage of evolution 

 almost identically parallel to our own. If we may reckon with 

 a thousand million suns, the chances are there may be ten 

 thousand such parallel worlds. 



Even this number is still enormous. It is quite beyond 

 the powers of the human mind vividly to picture the existence 



