CONTINUOUS VARIATIONS 281 



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. Bonnier 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 grown 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- 



