CONTINUOUS VARIATIONS 315 
different from that of the same species when growing 
in the plains, that inexperienced persons might readily 
suppose two such forms to belong to as many distinct 
species. At intermediate levels the habit is more or 
less intermediate. Bonniér made the experiment of 
dividing individual plants into two portions, plant- 
ing one part at a high elevation and the other near the 
level of the sea. In a few years the plant grown on 
the mountain had taken on the full alpine habit, 
whilst that growa on the plain retained the ordinary 
appearance of the species. In this way very con- 
siderable differences in habit were shown to be directly 
dependent on external conditions. 
In some few cases the environment determines the 
production of perfectly definite and discontinuous 
features. The water ranunculus, when growing sub- 
merged beneath the surface of a pond, produces leaves 
the blades of which are cut up into a great number of 
fine thread-like segments. As soon as the top of the 
plant reaches the surface of the water those leaf rudi- 
ments which are just commencing their existence pro- 
ceed to develop in a totally different fashion. The 
leaves to which they give rise possess a wide and undi- 
vided blade, which floats upon the surface of the water. 
The two sorts of leaves are as utterly different in 
appearance as it is possible for leaves to be. Yet the 
effect of external conditions upon the young leaf- 
rudiment determines which of the two kinds is to 
appear. 
In this instance we see a discontinuous change in 
conditions—the change from water to air as a sur- 
