THE COMING OF EVOLUTION 189 



of industry raised on that knowledge as a basis, which 

 most effectually widened man's mental horizon and 

 led to yet one more revolution in his ways of thought. 

 The point of real interest shifted from astronomy 

 to geology, and from physics to biology and the pheno- 

 mena of life. " Life," wrote Robert Hunter, the 

 great surgeon of the eighteenth century, " is a property 

 we do not understand. We can only see the necessary 

 steps leading to it." It was this property that men 

 now set themselves to examine critically. The neces- 

 sary steps began gradually to show signs of falling 

 into order in a scheme of natural knowledge, and 

 the theory of natural selection, which gave an accept- 

 able basis for the old and somewhat discredited 

 idea of evolution, carried the human mind over the 

 next long stage of its endless journey. Darwin was the 

 Newton of biology the central figure of nineteenth- 

 century thought. 



To trace the history and appreciate the significance 

 of evolutionary philosophy, we must follow the out- 

 lines of the growth in biological knowledge from the 

 point where we dropped the threads of its story in a 

 previous chapter. 



One of the principal lines of evidence in favour of 

 the theory of evolution has come from a comparison 



of animals, a branch of enquiry which in 

 Embryology. . . J 



the eighteenth century was in the hands 



of travellers and explorers ; but there is another 

 source of information which has proved no less con- 

 vincing to the trained scientific mind. The forma- 

 tive process through which every living thing, animal 

 or vegetable, passes before it takes up its final and 



