33° Evolution and Adaptation 



which, however much it may be distorted by external circun 

 stances, returns again to its original form as soon ^asTeleaset 



Besides these temporary changes, due to external influence 

 there are many cases known in which the same plant live 

 under very diverse conditions and yet remains exactly the sam< 

 For example, the species of Rhododendron ferragineum live 

 on archaean mountains and especially where the soil is poor i 

 calcium. Another species, Rhododendron hirsutum is foun 

 especially on soil rich in calcium. The difference in the tw 

 species has been supposed to depend on differences in th 

 soil, and if so, we would imagine that, if transplanted for 

 long time, the one should change in the direction of the othe 

 Yet it is known that the rusty rhododendron may be found i 

 all sorts of localities, even on dry, sunny, calcareous rocks c 

 the Apennines and of the Jura, and despite its residence i 

 these localities, since the glacial epoch, no change whateve 

 has taken place. 



Single varieties of the large and variable genus of Hien 

 cium have lived since the glacial period in the high regior 

 of the Alps, Carpathians, and in the far north, and also in th 

 plains of different geological formations, but these varietie 

 have remained exactly the same,, although on all sides ther 

 are transitional forms leading from these to other varieties. 



Some parasitic, species also furnish excellent illustrations c 

 the same principle. Besides the several species of Orobai 

 chia and of the parasitic moulds, the mistletoe deserves sped; 

 mention. It lives on both birch and apple trees and on bot 

 presents exactly the same appearance ; and even if it is tru 

 that mistletoe growing on conifers presents certain small dev 

 ations in its character, it is still doubtful whether, if tran: 

 ferred to the birch or apple tree, it would not lose thes 

 differences, thus indicating that they are not permanent. 



It is a fact of general observation that, on the one ham 

 the same variety occurs in different localities and under di 

 ferent surroundings, and, on the other hand, that slight! 



