VIII PREFACE. 



The advantage of the theory, here sketched out, seems to the author 

 to be, that it is based on the experimentally proven fact that crossing 

 gives rise to new forms, while, in as much as the real existence of here- 

 ditary variability is not proven, all theories based on variability have 

 a less solid basis. 



Unfortunately the final basis of evolution — the constitution of the 

 germplasma — remains as yet an utter mystery, so that we know next 

 to nothing of the final cause of evolution; the more reason to keep 

 facts and mere surmises well separated. This the author has aimed at, 

 and he flatters himself that, by so doing, he has cut a small peep-hole 

 in the curtain which hides the stage on which evolution takes its course. 

 Criticism within this contention, e. g. dealing with the part of evolution 

 which is discussed in this sketch, will indeed be most welcome to him 

 and will be considered in a careful manner and a grateful spirit. 

 Du choc des opinions jaillit la verity ! 



Haarlem, February 16th 1916. 



