314 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 
