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 ' Bet ween -Races J 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 



