PREFACE 



POST-DARWINIAN books on evolution 

 fall naturally into four classes. I. Those 

 which preach Wallaceism, as, for ex- 

 ample, Wallace's Darwinism, Poulton's 

 Essays on Evolution, and the voluminous works 

 of Weismann. 1 1. Those advocating Lamarckism. 

 Cope's Factors of Evolution and the writings of 

 Haeckel belong to this class. III. The writings 

 of De Vries, forming a group by themselves. 

 They advocate the theory that species spring 

 suddenly into being ; that new species arise by 

 mutations from pre-existing species. IV. The 

 large number of books of a more judicial nature, 

 books written by men who decline to subscribe 

 to any of the above three creeds. Excellent 

 examples of such works are Kellog's Darwinism 

 To-Day \ Lock's Recent Progress in the Study of 

 Variation, Heredity^ and Evolution, and T. H. 

 Morgan's Evolution and Adaptation. 



All four classes are characterised by defects. 



Books of the two first classes exhibit the 



faults of ardent partisanship. They formulate 



creeds, and, as Huxley truly remarked, " Science 



commits suicide when it adopts a creed." The 



