246 Non-Isolablc Races. 
must be brought about suddenly, and, under ordinary 
conditions of culture, be effected in the course of a few 
years. In this way the double variety may have arisen 
from time to time in the wild state; and in the same 
manner the present half race may perhaps, in the course 
of time, undergo this change. 
This transformation, however, cannot be simply the 
result of careful selection. A mutation is needed; and 
we know as little about the causes of mutations as about 
the method of inducing them artificially. Mutations are 
known to occur with moderate frequency both in breed- 
ing experiments and in nature, but, up to the present, 
their occurrence has been a matter of chance (10 and 
11, pp. 95-103). 
In my experiment such a mutation did not occur, 
although it extended over five generations. 1 The half 
race was distinctly improved by repeated and very strin- 
gent selection. It became at the end very rich in extreme 
or almost extreme variants, but it was just in these that 
it proved to be so remarkably constant. In its five gen- 
erations it reached a point which did not seem to me 
likely to be exceeded by further selection. It produced 
occasional flowers with more than 1 5 petals, and a single 
one with 31, but the mean number of the petals in its 
selected individuals did not exceed 9-10. 
The double variety did not arise from it, in spite of 
every effort. 
1 The fluctuating variability of the semi-latent character in Ranun- 
culus bulbosus scmiplctnis seems to cover a much wider range of 
forms than in Trifolium. There the extremes are 3 and 7 leaflets; 
in the buttercup they are 5 and 31 and perhaps more petals. From 
this it does not, however, follow that the variation is greater in the 
one case than in the other, but only that the variation is expressed 
by a larger number of divisions in the latter case, i. e., that there are 
more scale characters in the curve. 
