662 Geological Periods of Miifafioji. 



Koken's conception. In each of the several periods the 

 new forms appear in a swarm, whilst the periods them- 

 selves are separated by phases of rest. According to the 

 theory of selection the species themselves should be trans- 

 formed into new ones; but according to the theory of 

 mutation the original species does not disappear, whilst 

 the extremes press forward. In the case of Oenothera 

 Laiuarckiana the main stem continually multiplies with 

 undiminished vigor. Its derivate species have the greatest 

 difficulty in maintaining themselves in competition with 

 it in the natural state. Even so, as Koken points out, 

 paleontology recognizes numerous cases in which the type 

 species persists alongside those to which it has given rise, 

 and it may even sometimes persist after these have dis- 

 appeared. 



The genetic association of the individual types can 

 be demonstrated by experiment ; but in paleontology con- 

 clusions relating to this point must obviously be based 

 on considerations of a comparative nature. Apart from 

 this, everything seems to be exactly the same. *'The 

 swarms of varieties and species succeed one another like 

 the stories of a house. Similar forms recur bv bein"' 

 produced at various times by the conservative guardians 

 of the race, but not by one giving birth to the other."^ 

 Paleontolog}^ has the great advantage of directly demon- 

 strating the stories which follow one another, as such. 

 Comparative biology, on the other hand, has to infer 

 them from classification, whilst experiment will probably 

 always have to confine itself to a single story. 



According to the jM'ecedent set by Waagen, the sev- 

 eral forms of a group which follow one another in the 

 course of time, and by means of which a type is gradually 



* E. KoKEN^ Jahrh. d. k. k. gcol. Rcichsaiuts, [896, p. 40. 



