330 Evolution and Adaptation ‘ 
which, however much it may be distorted by external circum- 
stances, returns again to its original form as soon as released. 
Besides these temporary changes, due to external influences, 
there are many cases known in which the same plant lives 
under very diverse conditions and yet remains exactly the same. 
For example, the species of Rhododendron ferragineum lives 
on archzean mountains and especially where the soil is poor in 
calcium. Another species, Rhododendron hirsutum is found 
especially on soil rich in calcium. The difference in the two 
species has been supposed to depend on differences in the 
soil, and if so, we would imagine that, if transplanted for a 
long time, the one should change in the direction of the other. 
Yet it is known that the rusty rhododendron may be found in 
all sorts of localities, even on dry, sunny, calcareous rocks of 
the Apennines and of the Jura, and despite its residence in 
these localities, since the glacial epoch, no change whatever 
has taken place. 
Single varieties of the large and variable genus of Azera- 
cium have lived since the glacial period in the high regions 
of the Alps, Carpathians, and in the far north, and also in the 
plains of different geological formations, but these varieties 
have remained exactly the same, although on all sides there 
are transitional forms leading from these to other varieties. 
Some parasitic species also furnish excellent illustrations of 
the same principle. Besides the several species of Oroban- 
chia and of the parasitic moulds, the mistletoe deserves special 
mention. It lives on both birch and apple trees and on both 
presents exactly the same appearance; and even if it is true 
that mistletoe growing on conifers presents certain small devi- 
ations in its character, it is still doubtful whether, if trans- 
ferred to the birch or apple tree, it would not lose these 
differences, thus indicating that they are not permanent. 
It is a fact of general observation that, on the one hand, 
the same variety occurs in different localities and under dif- 
ferent surroundings, and, on the other hand, that slightly 
