Periodic Mutations 709 



the group called Oenothera, with a large num- 

 ber of species of the same general type as the 

 Owa.^ra-forms. Still farther downward comes 

 the old genus Oenothera itself, with numerous 

 subgenera diverging in sundry characters and 

 directions. 



Proceeding still farther we might easily con- 

 struct a main stem with numerous succeeding 

 fans of lateral branches, and thus reach, from 

 our new empirical point of view, the theoretical 

 conclusion already formulated. 



Paleontologic facts readily agree with this 

 conception. The swarms of species and va- 

 rieties are found to succeed one another like so 

 many stories. The same images are repeated, 

 and the single stories seem to be connected by 

 the main stems, which in each tier produce the 

 whole number of allied forms. Only a few pre- 

 vailing lines are prolonged through numerous 

 geologic periods; the vast majority of the lat- 

 eral branches are limited each to its own storey. 

 It is simply the extension of the pedigree of 

 the evening-primroses backward through ages, 

 with the same construction and the same lead- 

 ing features. There can be no doubt that we 

 are quite justified in assuming that evolution 

 has followed the same general laws through the 

 whole duration of life on earth. Only a mo- 

 ment of their lifetime is disclosed to us, but it 



