34 EVOLUTION AND NATURAL THEOLOGY. , 



most resembling it in. their physical character- 

 istics. The belt of asteroids, which some 

 astronomers suppose to be the fragments of l a 

 shattered planet, forms the boundary between 

 the smaller and larger planets. It is not thought 

 unlikely that some of the latter (Jupiter, for 

 example) are capable of radiating a sufficient 

 amount of light and heat to maintain life on 

 their satellites. This appears the more probable 

 when we remember that it is now known that 

 even the moon reflects a perceptible quantity of 

 heat, as well as light to the earth.* 



It has sometimes been argued,f but on very 

 insufficient grounds, that the earth is the only 

 planet in the Solar System, if not in the Uni- 

 verse, which is physically capable of supporting 

 life ; but, apart from the inherent improbability 

 of this theory,J the amount of light and heat 

 required by each planet for the use of its in- 

 habitants, is probably dependent as much on the 

 constitution of its atmosphere, and the amount 

 of its own internal heat, as on its actual distance 

 from the sun. Astronomers have supposed that 



* Piazzi Smith, "Teneriffe," p. 212. 

 t Whewell's " Plurality of Worlds." 

 | See Ponton, " The Beginning," ch. ix. 



