THEORY OF EVOLUTION 27 



which have made the process of evolution possi- 

 ble. And without this knowledge there can be 

 no understanding of the causes of evolution. 



THE FOUR GREAT HISTORICAL 

 SPECULATIONS 



Looking backward over tlie history of the 

 evolution theory we recognize that during the 

 hundred and odd years that have elapsed since 

 Buffon, there have been four main lines of 

 speculation concerning evolution. We might 

 call them the four great cosmogonies or the 

 four modern epics of evolution. 



The Exviroxment 



Geoff )-o?/ St. Hilaire 

 About the beginning of the last century 

 Cxeoffroy St. Hilaire, protege, and in some 

 respects a disciple of Buffon, was interested as 

 to how living species are related to the animals 

 and plants that had preceded them. He w^as 

 familiar with the kind of change that takes 

 place in the embryo if it is put into new or 

 changed surroundings, and from this knowl- 

 edge he concluded that as the surface of the 



