INTRODUCTION. 5 



well as for specific purity, soon saw that this theory was incom- 

 plete so long as it did not explain the causes of variation. Dif- 

 fering from Darwin in his attitude toward facts and Science, he 

 was so convinced of the power of selection, that he invented a 

 purely hypothetical intra-cellular struggle between "determi- 

 nants, germinal selection". 



De Vries, wanting to believe, that what he had witnessed in 

 Oenothera was the only way in which new species spring into 

 existence full-fledged, was confronted with two main^fficult- 

 ies. On one hand there were numerous instances,' in which, 

 among cultivated plants and animals selection on small differ- 

 ences had a permanent and far-reaching effect; on the othe^r 

 hand, what he had witnessed in one small group of plants did 

 not exist anywhere else. The first difficulty he ignored, and to 

 meet the second one, he had to invent purely hypothetical 

 "periods of mutability" and several minor h5?potheses as to 

 the internal causes of mutation. 



Finally, those authors, such as Lotsy, who want to believe 

 that crossing is the cause of species-formation, feel that cros- 

 sing, even though it may be the only cause of heriditable varia- 

 bility, does not explain specific stability. There is only one 

 course of action consistent with the wish to maintain crossing 

 in the role of "the" cause of species-formation, and that is to 

 deny variability within species. No Zoologist would deny the 

 existence of variation within species; the only way in which 

 a Botanist can do it, is Lotsy 's way, to take the term species 

 away from what everybody else calls species, and to give it to 

 those special species which exist in certain strictly autog- 

 amous plants, namely, the genetically pure groups of plants, 

 which everyone else calls "pure lines" after Johannsen. 



As long as it was possible, that finely wrought out hypothe- 

 ses about inheritance and variation were flatly opposed in 

 every important point, there did not exist a science of Genet- 

 ics, even if there did exist Geneticians. The fact that two such 

 absolutely opposed conceptions of the influence of selection 

 on species formation can exist at the same time, as that of 



