The Periodicity of Progressive Mutations. 653 
our species, have arisen suddenly, these changes must 
have been distributed more or less regularly over the 
whole line of ancestors of the Ocnothcra. How many 
steps are combined into a single period of mutation can- 
not be determined, and the question is obviously of sec- 
ondary interest only. The available evidence seems to 
indicate that only one step in the same direction occurs 
at one time ; but obviously this does not exclude the possi- 
bility of periods in which more numerous changes occur. 
In order to apply the results obtained with our prim- 
roses to earlier hypothetical periods of mutation, I will 
repeat the empirical pedigree of the first volume (p. 224), 
but in a somewhat different form. I will indicate the 
lateral branches which arise from the main stem in suc- 
cessive years, that is to say the new species, in the form 
of radiating groups (Fig. 148). Each group denotes the 
mutations in a single generation. The main stem contin- 
ues unchanged and successively produces the individual 
groups. Together, however, they obviously belong to one 
and the same period, inasmuch as each of them mainly 
consists of the same species and in approximately equal 
proportions. 
In order to compare this period with previous ones 
the whole figure may be compressed to a single group. 
This has been done in the upper part of Fig. 149. The 
lateral branches do not arise here from a single point, and 
this is intended to indicate the fact that the figure em- 
braces a series of generations in which the variations were 
repeated. 
As stated above, we will now assume that the ances- 
tors of our Ocnothcra have not always been mutable. 
Therefore our group must have a limit below, and must, 
so to speak, be borne by a stem without lateral branches. 
