2 THEORY OF EVOLUTION 



reaction within the mass until the chemical sub- 

 stances that we know today were produced. 

 This is the nebular hypothesis of the astrono- 

 mer. The astronomer explains, or tries to 

 explain, how this evolution took place, by an 

 appeal to the physical processes that have 

 been worked out in the laboratory, processes 

 which he tliinks have existed through all the 

 eons during which this evolution was going on 

 and which were its immediate causes. 



When the biologist thinks of the evolution 

 of animals and plants, a different picture pre- 

 sents itself. He thinks of series of animals 

 tliat have lived in the past, whose bones (fig. 

 1 ) and shells have been preserved in the rocks. 

 He thinks of these animals as having in the j)ast 

 given birtli. tln-ough an unbroken succession 

 of individuals, to tlie living inhabitants of the 

 earth today. He thinks that the old, simpler 

 types of the past have in j)art changed over into 

 the more complex forms of today. 



He is thinking as the historian thinks, but 

 he sometimes gets confused and thinks that he 

 is explaining evolution when he is only describ- 

 ing it. 



