662 Geological Periods of Mutation. 
KOKEN'S conception. In each of the several periods the 
new forms appear in a swarm, \vhilst 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 Ocnollicra 
Lainarckiana 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 by being- 
produced at various times by the conservative guardians 
of the race, but not by one giving birth to the other." 1 
Paleontology has the great advantage of directly demon- 
strating the stories which follow one another, as such. 
Comparative biology, on the other hand, has to in f ci- 
thern from classification, whilst experiment will probably 
always have to confine itself to a single story. 
According to the precedent 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 
1 E. KOKEX. Jahrb. d. k. k. gcol. Rcichsantts, 1896, p. 40. 
