150 THE THEORY OF MUTATION 
As a further illustration of what is meant by a 
between-race, mention may be made of the five-leaved 
race of purple clover (Trifolium pratense) obtained by 
de Vries, and developed by a process of selection. It 
would appear that the plants occasionally found grow- 
ing wild, which bear a single four-lobed leaf, usually 
belong only to a half-race. De Vries was fortunate 
enough to find two plants upon each of which several] 
of the leaves showed this anomaly, and from these, by 
an elaborate process of selection extending over several 
years, a race was obtained, the leaves of which in the 
majority of cases showed five lobes, whilst some had 
six or seven. Since, however, it appeared impossible 
to get rid of a certain proportion of three-lobed leaves, 
and equally so, on the other hand, to obtain leaves 
with more than seven lobes, de Vries concluded that 
his experiment exemplified the development of a mid- 
race, and not that of a constant race or true variety. 
The phenomena described by de Vries under the 
head of ‘ Between Races’ are difficult to understand 
until the reader has arrived at some appreciation of 
the very wide differences which even slight changes in 
external conditions may make in the growth and 
habit of plants. We may briefly describe one very 
striking example. 
The little water ranunculus, which often covers areas 
of stagnant water in the spring with a sheet of white 
blossom, possesses two kinds of leaves, the appearance 
of which entirely depends upon the stimulus of external 
circumstances. In the young state the whole plant is 
submerged beneath the surface of the water, and bears 
