6o 
THE PLANT WORLD. 
als.” No single parent plant of the Lamarckianas “proved ever to be 
wholly destitute of mutability.” 
6. Mutability and fluctuating variability are entirely distinct phenomena. 
7. “ Mutation takes place in nearly all directions.” There are two pos¬ 
sibilities as to how mutation changes may be brought about, and both 
were propounded by Darwin. “ One is the accumulation of the slight 
deviations of fluctuating variability, the other consists of successive sports 
or leaps taking place in the same direction.” 
“ The mutations of the evening-primrose comply with the demands 
made by Darwin as to the form of variability which is to be accepted as 
the cause of evolution and as the origin of species.” 
Origin of Wild Species and Varieties. —The origin of new elementary 
species of Oenothera is not an isolated instance, but occurs repeatedly in 
nature, e. g., in species of heather ( Erica and Calluna), Brunella, Ononis, 
Thymus, bilberries ( Vaccinium ), and campions (Lychnis). In consider¬ 
ing the problem of how to deal with these cases and what we may learn 
from them, three points are of first importance. First, to ascertain the 
constancy of the new type; second, to establish the occurrence or lack of 
transitional forms; third, to observe a repetition of the origin of the form 
by experimental pedigree-cultures. The first two points are not difficult 
to settle. Intermediates are never found. Two suppositions are possible 
—either they did exist but have perished or else they have never occurred. 
Since they are never found the supposition most warranted is that they 
do not occur. The many known cases which show no reversions nor even 
partial reversions combined with absence of intermediates “ makes the 
hypothesis of a slow and gradual development still more improbable.” 
Mutations have been observed in experimental pedigree-cultures in Cap- 
sella Heegeri and other plants from time to time and by different writers, 
e. g., cruciata forms of Oenothera and of Epilohium. Many other in¬ 
stances are recorded. One important point to be noted is the fact that 
the same novelty has been frequently found two or more times, “ and 
under conditions which make it very improbable that any relation between 
such occurrences might exist. The same mutation must have taken place 
more than once from the same main stem.” A striking example is the 
“ single-leaved ” strawberry which originated independently in Lapland, 
Versailles, and Holland. 
These considerations suggest that mutations are not very rare, and the 
reason they are not oftener met with is attributed to the fact that they 
perish in the struggle for existence. “ Thousands of mutations may per¬ 
haps take place yearly among the plants of our immediate vicinity with¬ 
out any chance of being discovered.” More will doubtless now be dis¬ 
covered when observations come to be made in the light of the mutation 
theory. There is here emphasized the “ lack of harmony between com¬ 
mon observations and the probable real state of things.” “ The chances 
of discovering new mutating species are great enough to justify the ut¬ 
most efforts to secure them.” 
