152 THE THEORY OF MUTATION 
flowers and the latter having white. The difference is 
that the former variety has the power of producing red 
flowers under certain circumstances which can be more 
or less rigidly defined, whilst the contrasted type has 
no such power. Thus, a few degrees difference in 
temperature may determine a marked change in the 
colour of a flower. 
It is not absolutely certain what it is that deter- 
mines the difference between the successive leaves on 
a plant of the five-leaved clover, but a strong hint is 
afforded by the fact that the leaves with the maximum 
number of lobes only appear when the plant is at the 
height of its vegetative activity. Early in the season, 
and again towards the close of the growing period, 
leaves with fewer lobes are produced. It would there- 
fore seem as if the change in the number of lobes were 
intimately connected with changes in the vigour and 
rate of growth of the plant. 
In the present chapter we are dealing particularly 
with the variations of plants, and, indeed, de Vries 
himself has never applied his views to the case of the 
animal kingdom. Although animals show some direct 
response in structure and functions to changes in their 
surroundings, these are not usually nearly so extensive 
or definite as the changes which we have just described. 
The statement has even been made that one of the 
fundamental differences between plants and the higher 
animals lies in the much greater susceptibility of the 
former to environmental changes. 
The views of de Vries with regard to the actual 
origin of new species may be summed up as follows: 
