42 THE THEORIES OF EVOLUTION 



For all these facts there can be only one explana- 

 tion: The embryo reproduces the appearance of an 

 ancestor common to a whole group of animals; the 

 variations which determined their differentiation only 

 appeared at a relatively later stage of embryonic 

 life and were, through inheritance, caused to appear 

 at approximately the same age. 



Neither can the survival of vestigial organs be ex- 

 plained by any theory except that of descent. 



Classification itself, which was still based upon tra- 

 ditionalist and non-transmutationist ideas, furnishes 

 further support to the transmutation theory. What 

 elements do we consider when we draw the line be- 

 tween the various groups? Not adapted organs, nor 

 similar organs, for we would then class the whale with 

 the fishes, but homologous organs and vestigial organs. 

 These are the elements which constitute the unity of 

 type, the great principle recognised by morphologists. 

 And unity of type merely symbolises actual relation- 

 ship between the species observed. 



Strengthened by all this evidence, Darwin finally 

 published his great work. We know what discus- 

 sions it brought forth, the enthusiasm of its partisans, 

 the violent hostility of its detractors. All the young 

 scientists athirst for progress sided with the new doc- 

 trine in its struggle against prejudice, religious tra- 

 ditions and the forces of reaction. 



In this first book, Darwin reserved, intentionally, 



