262 Heredity and Environment 



offers an extremely important means of quickly producing very 

 vigorous and fruitful individuals, but not lines or races which 

 breed true. 



2. Origin of Mutations. Mendelian association and dissocia- 

 tion of characters produces new forms of adult animals and 

 plants but not new hereditary characters. Permutations of Men- 

 delian characters we may have almost without number, of new 

 combinations of these there may be no end, but no new unit char- 

 acters are formed by such temporary combinations, no new in- 

 heritance factors are created or evolved. New combinations of 

 factors may be compared to new combinations of chemical ele- 

 ments ; you can always get out of the combination what went into 

 it, whereas new factors are comparable to the changes which take 

 place in certain atoms, for example radium, by which the element 

 itself is changed in an irreversible manner. The discoveries of 

 Mendel show us how to follow old characters through many com- 

 binations and through many generations, but they do not show us 

 how new characters arise. These discoveries have given us an 

 invaluable method of sorting and combining hereditary qualities, 

 but Mendelian inheritance as such does not furnish the materials 

 for evolution. 



In 1901 Hugo deVries startled the scientific world by the pub- 

 lication of his great work on the "Mutation Theory" of evolu- 

 tion in which he proved that the evening primrose, Oenothera 

 lamarckiana occasionally produced "sports" or "mutations" which 

 differed so much from the parent form that they deserved to be 

 called new species (Fig. 100). He discovered and studied a 

 large number of these mutations in Oenothera as well as in some 

 other plants and concluded that evolution takes place by steps 

 or jumps rather than by "creeping on from point to point" as 

 Darwin believed. 



Several genetecists have expressed doubt as to whether there 

 are any such things as mutations in the sense of deVries, 

 maintaining that all his results may be explained by assum- 

 ing that Oenothera is a hybrid and that the various "mutations" 



