Chapter I 



DIFFERENT KINDS OF EVOLUTION 



We use the word evolution in many ways — to in- 

 clude many different kinds of changes. There is 

 hardly any other scientific term that is used so care- 

 lessly — to imply so much, to mean so little. 



We S23eak of the evolution of the stars, of the evo- 

 lution of the horse, of the evolution of the steam 

 engine, as though they were all part of the same 

 process. What have they in common? Only this, 

 that each concerns itself with the history of some- 

 thing. When the astronomer thinks of the evo- 

 lution of the earth, the moon, the sun and the 

 stars, he has a picture of diffuse matter that has 

 slowly condensed. With condensation came heat; 

 with heat, action and reaction within the mass until 

 the chemical substances that we know today were 

 produced. This is the nebular hypothesis of the as- 

 tronomer. The astronomer explains, or tries to ex- 

 plain, how this evolution took place, by an appeal to 

 the physical processes that have been worked out in 

 the laboratory, processes which he thinks have ex- 

 isted 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 ani- 

 mals and plants, a different picture presents itself. 

 He thinks of series of animals that have lived in the 



