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 denned, 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 : 



