280 CONCLUDING CHAPTER 



the discontinuous nature of species in the following 

 manner : A great number of specific characters are, 

 without doubt, definite ; they are inherited as definite 

 entities, and there can be no question that their first 

 coming into existence was a definite event. Every 

 year tends to increase the range of characters to which 

 the conception of discontinuity has to be applied. 



Certain groups of characters do, however, seem to 

 exhibit the phenomena of continuity. Let us endeavour 

 to arrive at some closer idea as to the nature of these 

 characters. 



A study of continuous variations very quickly leads 

 to the conclusion that the variable features are those 

 which are especially liable to modification during the 

 lifetime of the individual, owing to the action of ex- 

 ternal circumstances. Such quantitative features of 

 size and shape and number of parts are particularly 

 plastic in the case of plants. 



The habit, or general form and appearance, of a 

 plant is a feature very characteristic of individual 

 species. The presence of a dwarf or of a tall habit 

 does, indeed, constitute a frequent distinction between 

 different strains of garden plants, and the inheritance 

 of these characters in many cases follows Mendel's 

 law. But leaving aside this particular example, the 

 inheritance of habit is very little understood ; although 

 habit is a feature which is very liable to considerable 

 fluctuations. Habit seems, in fact, usually to afford 

 an example of continuous variability. 



The habit of some species of plants when grown 

 under alpine conditions on mountain summits is so 



