42 GENETICS 



something present in the parental type, regressive 

 varieties. 



e. The same mutation may arise simultaneously in 

 many individuals instead of as a single "sport." 



f. Mutations do not vary around an arithmetical 

 mean with respect to the parent form, as is the case 

 with fluctuating variations, but each fluctuates around 

 a new average of its own, thus forming a discontinuous 

 series with the parent form. 



g. Mutations may occur in all directions, that is, 

 they are not necessarily definite or orthogenetic. 



h. Mutations probably appear periodically. 



i. Every mutation means two possible species where 

 one existed before. 



j. Useless or insignificant fluctuating variations are 

 not necessarily the material from which natural selec- 

 tion must sift out new species. 



k. Natural selection is not a causative agent in 

 evolutionary advance but is simply a sieve which picks 

 out successful survivors from mutations. 



3. LAMARCK'S EVENING PRIMROSE 



Perhaps the most widely known plant mutations are 

 the progeny of Lamarck's evening primrose, (Enothera 

 lamarckiana, because it was these plants that led 

 deVries to formulate his mutation theory. 



It is believed by botanists in general that this plant 

 is a native of the southern United States, although, 

 so far as is known, it is now extinct as a wild species 

 in America, and native specimens are included in but 

 few American herbaria. 



It was exported to London as a garden plant about 



